--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -2,17 +2,16 @@
  * 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/. */
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 @namespace svg url("http://www.w3.org/2000/svg");
 
 :root {
-  --identity-popup-expander-width: 38px;
   --panelui-subview-transition-duration: 150ms;
   --lwt-additional-images: none;
   --lwt-background-alignment: right top;
   --lwt-background-tiling: no-repeat;
 }
 
 :root:-moz-lwtheme {
   color: var(--lwt-text-color) !important;

--- a/browser/themes/shared/controlcenter/panel.inc.css
+++ b/browser/themes/shared/controlcenter/panel.inc.css
@@ -1,13 +1,15 @@
 %if 0
 /* 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/. */
 %endif
+%filter substitution
+%define identityPopupExpanderWidth 38px
 
 /* Hide all conditional elements by default. */
 :-moz-any([when-connection],[when-mixedcontent],[when-ciphers],[when-loginforms]) {
   display: none;
 }
 
 /* Show the right elements for the right connection states. */
 #identity-popup[connection=not-secure] [when-connection~=not-secure],
@@ -121,17 +123,17 @@
 }
 
 /* EXPAND BUTTON */
 
 .identity-popup-expander {
   margin: 0;
   padding: 4px 0;
   min-width: auto;
-  width: var(--identity-popup-expander-width);
+  width: @identityPopupExpanderWidth@;
   border-style: none;
   -moz-appearance: none;
   background: url("chrome://browser/skin/arrow-left.svg") center no-repeat;
   background-size: 16px, auto;
   -moz-context-properties: fill;
   fill: currentColor;
   color: inherit;
 }
@@ -189,17 +191,17 @@
   margin: 3px 0 4px;
   font-size: 150%;
 }
 
 .identity-popup-host {
   word-wrap: break-word;
   /* 1em + 2em + 24px is #identity-popup-security-content padding
    * 30em is .panel-mainview:not([panelid="PanelUI-popup"]) width */
-  max-width: calc(30rem - 3rem - 24px - var(--identity-popup-expander-width))
+  max-width: calc(30rem - 3rem - 24px - @identityPopupExpanderWidth@);
 }
 
 .identity-popup-warning-gray {
   padding-inline-start: 24px;
   background: url(chrome://browser/skin/controlcenter/warning-gray.svg) no-repeat 0 50%;
 }
 
 .identity-popup-warning-yellow {