--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -8,16 +8,17 @@
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 
 %include ../shared/browser.inc.css
 
 :root {
   --tabs-border: rgba(0,0,0,.3);
+  --tab-line-color: highlight;
 
   --toolbar-non-lwt-bgcolor: -moz-dialog;
   --toolbar-non-lwt-textcolor: -moz-dialogtext;
   --toolbar-non-lwt-bgimage: linear-gradient(rgba(255,255,255,.15), rgba(255,255,255,.15));
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
 
   --toolbarbutton-border-radius: 4px;

--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -9,16 +9,17 @@
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 
 %include ../shared/browser.inc.css
 
 :root {
   --tabs-border: rgba(0,0,0,.3);
+  --tab-line-color: #0a84ff;
 
   --toolbar-non-lwt-bgcolor: #f9f9fa;
   --toolbar-non-lwt-textcolor: #0c0c0d;
   --toolbar-non-lwt-bgimage: none;
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
 
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) + 1px);

--- a/browser/themes/shared/compacttheme.inc.css
+++ b/browser/themes/shared/compacttheme.inc.css
@@ -10,17 +10,17 @@
   --toolbar-bgcolor: var(--chrome-secondary-background-color);
   --toolbar-gbimage: none;
   --toolbar-non-lwt-bgcolor: var(--toolbar-bgcolor);
   --toolbar-non-lwt-textcolor: var(--chrome-color);
   --toolbar-non-lwt-bgimage: none;
 
   --toolbarbutton-icon-fill-opacity: .7;
 
-  --tab-line-color: highlight;
+  --tab-line-color: #0a84ff;
 }
 
 :root:-moz-lwtheme-brighttext {
   /* Chrome */
   --chrome-background-color: hsl(240, 5%, 5%);
   --chrome-color: rgb(249, 249, 250);
   --chrome-secondary-background-color: hsl(240, 1%, 20%);
   --toolbox-border-bottom-color: hsla(240, 5%, 5%, .1);

--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -3,17 +3,16 @@
  * 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/. */
 %endif
 %filter substitution
 %define horizontalTabPadding 9px
 
 :root {
   --tab-toolbar-navbar-overlap: 1px;
-  --tab-line-color: highlight;
   --tab-min-height: 33px;
   --tab-loading-fill: #0A84FF;
 }
 
 :root[uidensity=compact] {
   --tab-min-height: 29px;
 }
 

--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -8,16 +8,17 @@
 @namespace html url("http://www.w3.org/1999/xhtml");
 
 %include ../shared/browser.inc.css
 
 :root {
   --titlebar-text-color: currentColor;
 
   --tabs-border: threedshadow;
+  --tab-line-color: highlight;
 
   --toolbar-non-lwt-bgcolor: -moz-dialog;
   --toolbar-non-lwt-textcolor: -moz-dialogtext;
   --toolbar-non-lwt-bgimage: linear-gradient(rgba(255,255,255,.15), rgba(255,255,255,.15));
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
 
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) - 1px);
@@ -32,16 +33,17 @@
   --urlbar-separator-color: ThreeDLightShadow;
 
   --toolbox-border-bottom-color: ThreeDShadow;
 }
 
 @media (-moz-windows-default-theme) {
   :root {
     --tabs-border: rgba(0,0,0,.3);
+    --tab-line-color: #0a84ff;
 
     --toolbar-non-lwt-bgcolor: #f9f9fa;
     --toolbar-non-lwt-textcolor: #0c0c0d;
     --toolbar-non-lwt-bgimage: none;
 
     --toolbarbutton-icon-fill-opacity: .7;
 
     --panel-separator-color: hsla(210,4%,10%,.14);