Bug 1464714 - Make disabled .subviewbutton use --panel-disabled-color. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D4396

--HG--
extra : moz-landing-system : lando

browser/themes/shared/customizableui/panelUI.inc.css

@@ -871,6 +871,10 @@ panelview .toolbarbutton-1,
   outline: 0;
 }
 
+.subviewbutton[disabled="true"] {
+  color: var(--panel-disabled-color);
+}
+
 .subviewbutton > .toolbarbutton-text {
   padding: 0;
   padding-inline-start: 24px; /* This is 16px for the icon + 8px for the padding as defined below. */