--- a/mail/base/modules/ThemeVariableMap.jsm
+++ b/mail/base/modules/ThemeVariableMap.jsm
@@ -20,16 +20,19 @@ const ThemeVariableMap = [
     lwtProperty: "tab_loading",
   }],
   ["--lwt-tab-text", {
     lwtProperty: "tab_text"
   }],
   ["--tab-line-color", {
     lwtProperty: "tab_line",
   }],
+  ["--lwt-background-tab-separator-color", {
+    lwtProperty: "tab_background_separator",
+  }],
   ["--toolbar-bgcolor", {
     lwtProperty: "toolbarColor"
   }],
   ["--toolbar-color", {
     lwtProperty: "toolbar_text"
   }],
   ["--lwt-toolbar-field-border-color", {
     lwtProperty: "toolbar_field_border"

--- a/mail/themes/shared/mail/tabmail.css
+++ b/mail/themes/shared/mail/tabmail.css
@@ -193,17 +193,17 @@
   .tab-throbber {
     list-style-image: url("chrome://global/skin/icons/loading@2x.png");
   }
 }
 
 /* Tab separators */
 .tabmail-tab::after,
 .tabmail-tab::before {
-  border-left: 1px solid;
+  border-left: 1px solid var(--lwt-background-tab-separator-color, currentColor);
   margin-top: 5px;
   margin-bottom: 4px;
   opacity: 0.3;
 }
 
 /* Move the ::before pseudo-element on tabs 1px to the left
  * to avoid resizing the tab when the pseudo-element is removed again
  * (this currently happens when a tab is opened or closed).