fixup! Bug 40925: Implemented the Security Level component

Use --warning-color variable for the Custom label in the security level popup.


(cherry picked from commit 4f7b3857)

browser/components/securitylevel/content/securityLevelPanel.css

@@ -47,7 +47,7 @@ label#securityLevel-level {
 
 label#securityLevel-custom {
   border-radius: 4px;
-  background-color: var(--yellow-50);
+  background-color: var(--warning-color);
   color: black;
   font-size: 1em;
   height: 1.6em;