--- a/browser/components/extensions/ext-utils.js
+++ b/browser/components/extensions/ext-utils.js
@@ -4,16 +4,17 @@
 
 XPCOMUtils.defineLazyModuleGetter(this, "CustomizableUI",
                                   "resource:///modules/CustomizableUI.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
                                   "resource://gre/modules/PrivateBrowsingUtils.jsm");
 
 Cu.import("resource://gre/modules/ExtensionUtils.jsm");
 Cu.import("resource://gre/modules/AddonManager.jsm");
+Cu.import("resource://gre/modules/AppConstants.jsm");
 
 const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
 const INTEGER = /^[1-9]\d*$/;
 
 var {
   EventManager,
   instanceOf,
@@ -133,16 +134,35 @@ function promisePopupShown(popup) {
       popup.addEventListener("popupshown", function onPopupShown(event) {
         popup.removeEventListener("popupshown", onPopupShown);
         resolve();
       });
     }
   });
 }
 
+XPCOMUtils.defineLazyGetter(global, "stylesheets", () => {
+  let styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"]
+      .getService(Components.interfaces.nsIStyleSheetService);
+  let styleSheetURI = Services.io.newURI("chrome://browser/content/extension.css",
+                                         null, null);
+  let styleSheet = styleSheetService.preloadSheet(styleSheetURI,
+                                                  styleSheetService.AGENT_SHEET);
+  let stylesheets = [styleSheet];
+
+  if (AppConstants.platform === "macosx") {
+    styleSheetURI = Services.io.newURI("chrome://browser/content/extension-mac.css",
+                                       null, null);
+    let macStyleSheet = styleSheetService.preloadSheet(styleSheetURI,
+                                                       styleSheetService.AGENT_SHEET);
+    stylesheets.push(macStyleSheet);
+  }
+  return stylesheets;
+});
+
 class BasePopup {
   constructor(extension, viewNode, popupURL) {
     let popupURI = Services.io.newURI(popupURL, null, extension.baseURI);
 
     Services.scriptSecurityManager.checkLoadURIWithPrincipal(
       extension.principal, popupURI,
       Services.scriptSecurityManager.DISALLOW_SCRIPT);
 
@@ -158,16 +178,17 @@ class BasePopup {
     this.viewNode.addEventListener(this.DESTROY_EVENT, this);
 
     this.browser = null;
     this.browserReady = this.createBrowser(viewNode, popupURI);
   }
 
   destroy() {
     this.browserReady.then(() => {
+      this.browser.removeEventListener("DOMWindowCreated", this, true);
       this.browser.removeEventListener("load", this, true);
       this.browser.removeEventListener("DOMTitleChanged", this, true);
       this.browser.removeEventListener("DOMWindowClose", this, true);
 
       this.viewNode.removeEventListener(this.DESTROY_EVENT, this);
 
       this.context.unload();
       this.browser.remove();
@@ -185,16 +206,23 @@ class BasePopup {
   }
 
   handleEvent(event) {
     switch (event.type) {
       case this.DESTROY_EVENT:
         this.destroy();
         break;
 
+      case "DOMWindowCreated":
+        let winUtils = this.browser.contentWindow
+            .QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils);
+        for (let stylesheet of global.stylesheets) {
+          winUtils.addSheet(stylesheet, winUtils.AGENT_SHEET);
+        }
+        break;
       case "DOMWindowClose":
         if (event.target === this.browser.contentWindow) {
           event.preventDefault();
           this.closePopup();
         }
         break;
 
       case "DOMTitleChanged":
@@ -212,17 +240,16 @@ class BasePopup {
         // dynamically, probably in response to MozScrolledAreaChanged events.
         this.window.setTimeout(() => this.resizeBrowser(), 0);
         break;
     }
   }
 
   createBrowser(viewNode, popupURI) {
     let document = viewNode.ownerDocument;
-
     this.browser = document.createElementNS(XUL_NS, "browser");
     this.browser.setAttribute("type", "content");
     this.browser.setAttribute("disableglobalhistory", "true");
 
     // Note: When using noautohide panels, the popup manager will add width and
     // height attributes to the panel, breaking our resize code, if the browser
     // starts out smaller than 30px by 10px. This isn't an issue now, but it
     // will be if and when we popup debugging.
@@ -254,16 +281,17 @@ class BasePopup {
         contentWindow,
         uri: popupURI,
         docShell: this.browser.docShell,
       });
 
       GlobalManager.injectInDocShell(this.browser.docShell, this.extension, this.context);
       this.browser.setAttribute("src", this.context.uri.spec);
 
+      this.browser.addEventListener("DOMWindowCreated", this, true);
       this.browser.addEventListener("load", this, true);
       this.browser.addEventListener("DOMTitleChanged", this, true);
       this.browser.addEventListener("DOMWindowClose", this, true);
     });
   }
 
   // Resizes the browser to match the preferred size of the content.
   resizeBrowser() {

new file mode 100644
--- /dev/null
+++ b/browser/components/extensions/extension-mac.css
@@ -0,0 +1,11 @@
+button,
+select,
+input[type="checkbox"] + label::before {
+  border-radius: 4px;
+}
+
+.panel-section-footer {
+  border-bottom-left-radius: 4px;
+  border-bottom-right-radius: 4px;
+  overflow: hidden;
+}

new file mode 100644
--- /dev/null
+++ b/browser/components/extensions/extension.css
@@ -0,0 +1,572 @@
+/* stylelint-disable property-no-vendor-prefix */
+/* stylelint-disable property-no-vendor-prefix */
+/* Base */
+button,
+select,
+option,
+input {
+  -moz-appearance: none;
+}
+
+/* Variables */
+html,
+body {
+  background-color: #fcfcfc;
+  box-sizing: border-box;
+  color: #222426;
+  cursor: default;
+  display: flex;
+  flex-direction: column;
+  font: caption;
+  margin: 0;
+  padding: 0;
+  -moz-user-select: none;
+}
+
+body * {
+  box-sizing: border-box;
+  text-align: start;
+}
+
+/* stylelint-disable property-no-vendor-prefix */
+/* Buttons */
+button,
+select {
+  background-color: #fbfbfb;
+  border: 1px solid #b1b1b1;
+  box-shadow: 0 0 0 0 transparent;
+  font: caption;
+  height: 24px;
+  outline: 0 !important;
+  padding: 0 8px 0;
+  transition-duration: 250ms;
+  transition-property: box-shadow, border;
+}
+
+select {
+  background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiIgdmlld0JveD0iMCAwIDE2IDE2Ij4KICA8cGF0aCBkPSJNOCwxMkwzLDcsNCw2bDQsNCw0LTQsMSwxWiIgZmlsbD0iIzZBNkE2QSIgLz4KPC9zdmc+Cg==);
+  background-position: calc(100% - 4px) center;
+  background-repeat: no-repeat;
+  -moz-padding-end: 24px;
+  text-overflow: ellipsis;
+}
+
+label {
+  font: caption;
+}
+
+button::-moz-focus-inner {
+  border: 0;
+  outline: 0;
+}
+
+/* Dropdowns */
+select {
+  background-color: #fbfbfb;
+  border: 1px solid #b1b1b1;
+  box-shadow: 0 0 0 0 transparent;
+  font: caption;
+  height: 24px;
+  outline: 0 !important;
+  padding: 0 8px 0;
+  transition-duration: 250ms;
+  transition-property: box-shadow, border;
+}
+
+select {
+  background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiIgdmlld0JveD0iMCAwIDE2IDE2Ij4KICA8cGF0aCBkPSJNOCwxMkwzLDcsNCw2bDQsNCw0LTQsMSwxWiIgZmlsbD0iIzZBNkE2QSIgLz4KPC9zdmc+Cg==);
+  background-position: calc(100% - 4px) center;
+  background-repeat: no-repeat;
+  -moz-padding-end: 24px;
+  text-overflow: ellipsis;
+}
+
+select:-moz-focusring {
+  color: transparent;
+  text-shadow: 0 0 0 #000;
+}
+
+select:-moz-focusring * {
+  color: #000;
+  text-shadow: none;
+}
+
+button.hover,
+select.hover {
+  background-color: #ebebeb;
+  border: 1px solid #b1b1b1;
+}
+
+button.pressed,
+select.pressed {
+  background-color: #d4d4d4;
+  border: 1px solid #858585;
+}
+
+button.disabled,
+select.disabled {
+  color: #999;
+  opacity: .5;
+}
+
+button.focused,
+select.focused {
+  border-color: #fff;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+button.default {
+  background-color: #0996f8;
+  border-color: #0670cc;
+  color: #fff;
+}
+
+button.default.hover {
+  background-color: #0670cc;
+  border-color: #005bab;
+}
+
+button.default.pressed {
+  background-color: #005bab;
+  border-color: #004480;
+}
+
+button.default.focused {
+  border-color: #fff;
+}
+
+/* Radio Buttons */
+.radioItem {
+  margin-bottom: 6px;
+  text-align: left;
+}
+
+input[type="radio"] {
+  display: none;
+}
+
+input[type="radio"] + label {
+  -moz-user-select: none;
+}
+
+input[type="radio"] + label::before {
+  background-color: #fff;
+  background-position: center;
+  border: 1px solid #b1b1b1;
+  border-radius: 50%;
+  content: "";
+  display: inline-block;
+  height: 16px;
+  margin-right: 6px;
+  vertical-align: text-top;
+  width: 16px;
+}
+
+input[type="radio"]:hover + label::before,
+.radioItem.hover input[type="radio"]:not(active) + label::before {
+  background-color: #fbfbfb;
+  border-color: #b1b1b1;
+}
+
+input[type="radio"]:hover:active + label::before,
+.radioItem.pressed input[type="radio"]:not(active) + label::before {
+  background-color: #ebebeb;
+  border-color: #858585;
+}
+
+input[type="radio"]:checked + label::before {
+  background-color: #0996f8;
+  background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiIgdmlld0JveD0iMCAwIDE2IDE2Ij4KICA8Y2lyY2xlIGN4PSI4IiBjeT0iOCIgcj0iNCIgZmlsbD0iI2ZmZiIgLz4KPC9zdmc+Cg==);
+  border-color: #0670cc;
+}
+
+input[type="radio"]:checked:hover + label::before,
+.radioItem.hover input[type="radio"]:checked:not(active) + label::before {
+  background-color: #0670cc;
+  border-color: #005bab;
+}
+
+input[type="radio"]:checked:hover:active + label::before,
+.radioItem.pressed input[type="radio"]:checked:not(active) + label::before {
+  background-color: #005bab;
+  border-color: #004480;
+}
+
+.radioItem.disabled input[type="radio"] + label,
+.radioItem.disabled input[type="radio"]:hover + label,
+.radioItem.disabled input[type="radio"]:hover:active + label {
+  color: #999;
+  opacity: .5;
+}
+
+.radioItem.focused input[type="radio"] + label::before {
+  border-color: #0996f8;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+.radioItem.focused input[type="radio"]:checked + label::before {
+  border-color: #fff;
+}
+
+/* Checkboxes */
+.checkboxItem {
+  margin-bottom: 6px;
+  text-align: left;
+}
+
+input[type="checkbox"] {
+  display: none;
+}
+
+input[type="checkbox"] + label {
+  -moz-user-select: none;
+}
+
+input[type="checkbox"] + label::before {
+  background-color: #fff;
+  background-position: center;
+  border: 1px solid #b1b1b1;
+  content: "";
+  display: inline-block;
+  height: 16px;
+  margin-right: 6px;
+  vertical-align: text-top;
+  width: 16px;
+}
+
+input[type="checkbox"]:hover + label::before,
+.checkboxItem.hover input[type="checkbox"]:not(active) + label::before {
+  background-color: #fbfbfb;
+  border-color: #b1b1b1;
+}
+
+input[type="checkbox"]:hover:active + label::before,
+.checkboxItem.pressed input[type="checkbox"]:not(active) + label::before {
+  background-color: #ebebeb;
+  border-color: #858585;
+}
+
+input[type="checkbox"]:checked + label::before {
+  background-color: #0996f8;
+  background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiIgdmlld0JveD0iMCAwIDE2IDE2Ij4KICA8cGF0aCBkPSJNNy43LDEyLjkgQzcuNCwxMy4zIDYuOCwxMy40IDYuNCwxMyBMMy4yLDkuOCBDMi44LDkuNCAyLjgsOC42IDMuMiw4LjIgQzMuNiw3LjggNC40LDcuOCA0LjgsOC4yIEw2LjksMTAuMyBMMTEuMSw0LjQgQzExLjUsMy45IDEyLjIsMy44IDEyLjcsNC4xIEMxMy4yLDQuNSAxMy4zLDUuMiAxMyw1LjcgTDcuNywxMi45IEw3LjcsMTIuOSBaIiBmaWxsPSIjZmZmIiAvPgo8L3N2Zz4K);
+  border-color: #0670cc;
+}
+
+input[type="checkbox"]:checked:hover + label::before,
+.checkboxItem.hover input[type="checkbox"]:checked:not(active) + label::before {
+  background-color: #0670cc;
+  border-color: #005bab;
+}
+
+input[type="checkbox"]:checked:hover:active + label::before,
+.checkboxItem.pressed input[type="checkbox"]:checked:not(active) + label::before {
+  background-color: #005bab;
+  border-color: #004480;
+}
+
+.checkboxItem.disabled input[type="checkbox"] + label,
+.checkboxItem.disabled input[type="checkbox"]:hover + label,
+.checkboxItem.disabled input[type="checkbox"]:hover:active + label {
+  color: #999;
+  opacity: .5;
+}
+
+.checkboxItem.focused input[type="checkbox"] + label::before {
+  border-color: #0996f8;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+.checkboxItem.focused input[type="checkbox"]:checked + label::before {
+  border-color: #fff;
+}
+
+/* Expander Button */
+button.expander {
+  background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiIgdmlld0JveD0iMCAwIDE2IDE2Ij4KICA8cGF0aCBkPSJNOCwxMkwzLDcsNCw2bDQsNCw0LTQsMSwxWiIgZmlsbD0iIzZBNkE2QSIgLz4KPC9zdmc+Cg==);
+  background-position: center;
+  background-repeat: no-repeat;
+  height: 24px;
+  padding: 0;
+  width: 24px;
+}
+
+/* Interactive States */
+button:hover:not(.pressed):not(.disabled):not(.focused),
+select:hover:not(.pressed):not(.disabled):not(.focused) {
+  background-color: #ebebeb;
+  border: 1px solid #b1b1b1;
+}
+
+button:hover:active:not(.hover):not(.disabled):not(.focused),
+select:hover:active:not(.hover):not(.disabled):not(.focused) {
+  background-color: #d4d4d4;
+  border: 1px solid #858585;
+}
+
+button.default:hover:not(.pressed):not(.disabled):not(.focused) {
+  background-color: #0670cc;
+  border-color: #005bab;
+}
+
+button.default:hover:active:not(.hover):not(.disabled):not(.focused) {
+  background-color: #005bab;
+  border-color: #004480;
+}
+
+button:focus:not(.disabled) {
+  border-color: #fff !important;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+/* Fields */
+input[type="text"],
+textarea {
+  background-color: #fff;
+  border: 1px solid #b1b1b1;
+  box-shadow: 0 0 0 0 rgba(97, 181, 255, 0);
+  font: caption;
+  padding: 0 6px 0;
+  transition-duration: 250ms;
+  transition-property: box-shadow;
+}
+
+input[type="text"] {
+  height: 24px;
+}
+
+input[type="text"].hover,
+textarea.hover {
+  border: 1px solid #858585;
+}
+
+input[type="text"].disabled,
+textarea.disabled {
+  color: #999;
+  opacity: .5;
+}
+
+input[type="text"].focused,
+textarea.focused {
+  border-color: #0996f8;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+/* Interactive States */
+input[type="text"]:not(disabled):hover,
+textarea:not(disabled):hover {
+  border: 1px solid #858585;
+}
+
+input[type="text"]:focus,
+input[type="text"]:focus:hover,
+textarea:focus,
+textarea:focus:hover {
+  border-color: #0996f8;
+  box-shadow: 0 0 0 2px rgba(97, 181, 255, 0.75);
+}
+
+/* stylelint-disable property-no-vendor-prefix */
+.panel-section {
+  display: flex;
+  flex-direction: row;
+}
+
+.panel-section-separator {
+  background-color: rgba(0, 0, 0, 0.15);
+  min-height: 1px;
+}
+
+/* Panel Section - Header */
+.panel-section-header {
+  border-bottom: 1px solid rgba(0, 0, 0, 0.15);
+  padding: 16px;
+}
+
+.panel-section-header > .icon-section-header {
+  background-position: center center;
+  background-repeat: no-repeat;
+  height: 32px;
+  margin-right: 16px;
+  position: relative;
+  width: 32px;
+}
+
+.panel-section-header > .text-section-header {
+  align-self: center;
+  font-size: 1.385em;
+  font-weight: lighter;
+}
+
+/* Panel Section - List */
+.panel-section-list {
+  flex-direction: column;
+  padding: 4px 0;
+}
+
+.panel-list-item {
+  align-items: center;
+  display: flex;
+  flex-direction: row;
+  height: 24px;
+  padding: 0 16px;
+}
+
+.panel-list-item:not(.disabled):hover {
+  background-color: rgba(0, 0, 0, 0.06);
+  border-bottom: 1px solid rgba(0, 0, 0, 0.1);
+  border-top: 1px solid rgba(0, 0, 0, 0.1);
+}
+
+.panel-list-item:not(.disabled):hover:active {
+  background-color: rgba(0, 0, 0, 0.1);
+}
+
+.panel-list-item.disabled {
+  color: #999;
+}
+
+.panel-list-item > .icon {
+  flex-grow: 0;
+  flex-shrink: 0;
+}
+
+.panel-list-item > .text {
+  flex-grow: 10;
+}
+
+.panel-list-item > .text-shortcut {
+  color: #808080;
+  font-family: "Lucida Grande", caption;
+  font-size: .847em;
+  justify-content: flex-end;
+}
+
+.panel-section-list .panel-section-separator {
+  margin: 4px 0;
+}
+
+/* Panel Section - Form Elements */
+.panel-section-formElements {
+  display: flex;
+  flex-direction: column;
+  padding: 16px;
+}
+
+.panel-formElements-item {
+  align-items: center;
+  display: flex;
+  flex-direction: row;
+  margin-bottom: 12px;
+}
+
+.panel-formElements-item:last-child {
+  margin-bottom: 0;
+}
+
+.panel-formElements-item label {
+  flex-shrink: 0;
+  margin-right: 6px;
+  text-align: right;
+}
+
+.panel-formElements-item input[type="text"],
+.panel-formElements-item select {
+  flex-grow: 1;
+}
+
+/* Panel Section - Footer */
+.panel-section-footer {
+  background-color: rgba(0, 0, 0, 0.06);
+  border-top: 1px solid rgba(0, 0, 0, 0.15);
+  color: #1a1a1a;
+  display: flex;
+  flex-direction: row;
+  height: 41px;
+  margin-top: -1px;
+  padding: 0;
+}
+
+.panel-section-footer-button {
+  flex: 1 1 auto;
+  height: 100%;
+  margin: 0 -1px;
+  padding: 12px;
+  text-align: center;
+}
+
+.panel-section-footer-button > .text-shortcut {
+  color: #808080;
+  font-family: "Lucida Grande", caption;
+  font-size: .847em;
+}
+
+.panel-section-footer-button:hover {
+  background-color: rgba(0, 0, 0, 0.06);
+}
+
+.panel-section-footer-button:hover:active {
+  background-color: rgba(0, 0, 0, 0.1);
+}
+
+.panel-section-footer-button.default {
+  background-color: #0996f8;
+  box-shadow: 0 1px 0 #0670cc inset;
+  color: #fff;
+}
+
+.panel-section-footer-button.default:hover {
+  background-color: #0670cc;
+  box-shadow: 0 1px 0 #005bab inset;
+}
+
+.panel-section-footer-button.default:hover:active {
+  background-color: #005bab;
+  box-shadow: 0 1px 0 #004480 inset;
+}
+
+.panel-section-footer-separator {
+  background-color: rgba(0, 0, 0, 0.1);
+  width: 1px;
+  z-index: 99;
+}
+
+/* Panel Section - Tabs */
+.panel-section-tabs {
+  color: #1a1a1a;
+  display: flex;
+  flex-direction: row;
+  height: 41px;
+  margin-bottom: -1px;
+  padding: 0;
+}
+
+.panel-section-tabs-button {
+  flex: 1 1 auto;
+  height: 100%;
+  margin: 0 -1px;
+  padding: 12px;
+  text-align: center;
+}
+
+.panel-section-tabs-button:hover {
+  background-color: rgba(0, 0, 0, 0.06);
+}
+
+.panel-section-tabs-button:hover:active {
+  background-color: rgba(0, 0, 0, 0.1);
+}
+
+.panel-section-tabs-button.selected {
+  box-shadow: 0 -1px 0 #0670cc inset, 0 -4px 0 #0996f8 inset;
+  color: #0996f8;
+}
+
+.panel-section-tabs-button.selected:hover {
+  color: #0670cc;
+}
+
+.panel-section-tabs-separator {
+  background-color: rgba(0, 0, 0, 0.1);
+  width: 1px;
+  z-index: 99;
+}

--- a/browser/components/extensions/jar.mn
+++ b/browser/components/extensions/jar.mn
@@ -1,13 +1,15 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 browser.jar:
+    content/browser/extension.css
+    content/browser/extension-mac.css
     content/browser/extension.svg
     content/browser/ext-utils.js
     content/browser/ext-commands.js
     content/browser/ext-contextMenus.js
     content/browser/ext-browserAction.js
     content/browser/ext-pageAction.js
     content/browser/ext-desktop-runtime.js
     content/browser/ext-tabs.js

--- a/browser/components/extensions/test/browser/browser_ext_browserAction_popup.js
+++ b/browser/components/extensions/test/browser/browser_ext_browserAction_popup.js
@@ -1,29 +1,35 @@
 /* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
 /* vim: set sts=2 sw=2 et tw=80: */
 "use strict";
 
 function* testInArea(area) {
-  let scriptPage = url => `<html><head><meta charset="utf-8"><script src="${url}"></script></head></html>`;
+  let scriptPage = url => `<html><head><meta charset="utf-8"><script src="${url}"></script></head><body>${url}</body></html>`;
 
   let extension = ExtensionTestUtils.loadExtension({
     manifest: {
       "background": {
         "page": "data/background.html",
       },
       "browser_action": {
         "default_popup": "popup-a.html",
       },
     },
 
     files: {
       "popup-a.html": scriptPage("popup-a.js"),
       "popup-a.js": function() {
-        browser.runtime.sendMessage("from-popup-a");
+        window.onload = () => {
+          if (window.getComputedStyle(document.body).backgroundColor == "rgb(252, 252, 252)") {
+            browser.runtime.sendMessage("from-popup-a");
+          } else {
+            browser.runtime.sendMessage("popup-a-failed-style-check");
+          }
+        };
         browser.runtime.onMessage.addListener(msg => {
           if (msg == "close-popup") {
             window.close();
           }
         });
       },
 
       "data/popup-b.html": scriptPage("popup-b.js"),
@@ -69,16 +75,18 @@ function* testInArea(area) {
         sendClick = ({expectEvent, expectPopup, runNextTest}) => {
           expect = {event: expectEvent, popup: expectPopup, runNextTest};
           browser.test.sendMessage("send-click");
         };
 
         browser.runtime.onMessage.addListener(msg => {
           if (msg == "close-popup") {
             return;
+          } else if (msg == "popup-a-failed-style-check") {
+            browser.test.fail("popup failed style check");
           } else if (expect.popup) {
             browser.test.assertEq(msg, `from-popup-${expect.popup}`,
                                   "expected popup opened");
           } else {
             browser.test.fail(`unexpected popup: ${msg}`);
           }
 
           expect.popup = null;

--- a/browser/themes/shared/customizableui/panelUIOverlay.inc.css
+++ b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
@@ -250,16 +250,24 @@ panelmultiview[nosubviews=true] > .panel
   max-width: @standaloneSubviewWidth@;
 }
 
 /* Give WebExtension stand-alone panels extra width for Chrome compatibility */
 .cui-widget-panel[viewId^=PanelUI-webext-] .panel-mainview {
   max-width: 800px;
 }
 
+.cui-widget-panel[viewId^=PanelUI-webext-] > .panel-arrowcontainer > .panel-arrowcontent {
+  padding: 0;
+}
+
+.cui-widget-panelview[id^=PanelUI-webext-] {
+  border-radius: 3.5px;
+}
+
 panelview:not([mainview]) .toolbarbutton-text,
 .cui-widget-panel toolbarbutton > .toolbarbutton-text {
   text-align: start;
   display: -moz-box;
 }
 
 .cui-widget-panel > .panel-arrowcontainer > .panel-arrowcontent {
   padding: 4px 0;