Bug 1379122 - Fix UI density menuitem font color on Linux. r=dao

MozReview-Commit-ID: G3CULzSXZzz

--HG--
extra : rebase_source : 101f1b91c6c96d23416c8835188e5387de0289e6

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

@@ -475,6 +475,10 @@ toolbarpaletteitem[place=toolbar] > toolbarspring {
   padding-inline-start: 0;
 }
 
+.customization-uidensity-menu-button {
+  color: inherit;
+}
+
 .customization-lwtheme-menu-theme[defaulttheme] {
   list-style-image: url(chrome://browser/content/default-theme-icon.svg);
 }