Bug 1854315 - Make popups not inherit container color scheme. r=dao,desktop-theme-reviewers

The `menubar` has bright text and thus we use `color-scheme: dark` on it
to get the right colors.

But popups should generally be able to render on the preferred color
scheme.

Differential Revision: https://phabricator.services.mozilla.com/D189025

toolkit/themes/shared/popup.css

@@ -8,6 +8,10 @@
 
 menupopup,
 panel {
+  /* We can always render in the preferred color scheme (unless otherwise
+   * overridden). */
+  color-scheme: light dark;
+
   min-width: 1px;
   --panel-background: Menu;
   --panel-color: MenuText;