Bug 1785155 - Make optgroup background be reflected on Linux with custom select styling. r=dholbert

This matches Windows' behavior with this pref enabled, though I'd note
that might not be the reporter's expectation, since we only style the
caption, which is a sibling rather than an ancestor of the menu items.

That might need further fixes, anyways seems worth doing this
regardless.

Differential Revision: https://phabricator.services.mozilla.com/D154838

toolkit/themes/linux/global/menu.css

@@ -41,7 +41,8 @@ menuitem[_moz-menuactive="true"] {
   background-color: -moz-menuhover;
 }
 
-menuitem[customoptionstyling="true"] {
+menuitem[customoptionstyling="true"],
+menucaption[customoptionstyling="true"] {
   appearance: none;
 }