diff --git a/browser/themes/osx/customizableui/panelUIOverlay.css b/browser/themes/osx/customizableui/panelUIOverlay.css
index 4eacb5151b4c..54a016d9df52 100644
--- a/browser/themes/osx/customizableui/panelUIOverlay.css
+++ b/browser/themes/osx/customizableui/panelUIOverlay.css
@@ -4,10 +4,6 @@
 
 %include ../../shared/customizableui/panelUIOverlay.inc.css
 
-:root {
-  --panel-separator-color: hsla(210,4%,10%,.15);
-}
-
 .panel-subviews {
   background-color: hsla(0,0%,100%,.97);
 }
@@ -77,4 +73,4 @@ menu.subviewbutton > .menu-right > image {
 
 toolbarpaletteitem[place="palette"] > .toolbarbutton-1 > .toolbarbutton-menubutton-button {
   padding: 3px 1px;
-}
+}
\ No newline at end of file
diff --git a/browser/themes/shared/customizableui/panelUIOverlay.inc.css b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
index 365db4b0909d..fc5f7a873082 100644
--- a/browser/themes/shared/customizableui/panelUIOverlay.inc.css
+++ b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
@@ -21,7 +21,6 @@
 %include ../browser.inc
 
 :root {
-  --panel-separator-color: ThreeDShadow;
   --panel-ui-exit-subview-gutter-width: 38px;
 }
 
@@ -1154,8 +1153,7 @@ menuitem.panel-subview-footer@menuStateActive@,
 .cui-widget-panelview menuseparator {
   -moz-appearance: none;
   min-height: 0;
-  border-top: 1px solid var(--panel-separator-color);
-  border-bottom: none;
+  border-top: 1px solid hsla(210,4%,10%,.15);
   margin: 6px 0;
   padding: 0;
 }
@@ -1348,7 +1346,7 @@ toolbarpaletteitem[haswideitem][place="panel"] + toolbarpaletteitem[haswideitem]
   margin: .5em 0;
   width: 1px;
   height: auto;
-  background: var(--panel-separator-color);
+  background: hsla(210,4%,10%,.15);
   transition-property: margin;
   transition-duration: 10ms;
   transition-timing-function: ease;