[web] Use themed colour for rail separator (#31269)

GitOrigin-RevId: e7db2f7d33f07702940fce8b3a2f725914a421f8

services/web/frontend/stylesheets/pages/editor/rail.scss

@@ -213,7 +213,7 @@ body {
 .separated-rail-tab::before {
   display: inline-block;
   content: '';
-  border-top: 1px solid var(--border-divider);
+  border-top: 1px solid var(--border-divider-themed);
   width: 100%;
   transform: translateY(-12px);
 }