Bug 1612509 - Remove custom colour for autocomplete-popup-highlight-background in Dark Mode. r=dao

I opted to remove this colour altogether instead of adding branching logic for the legacy Urlbar and the search bar. The legacy Urlbar is being replaced and this patch will land in the same version, and the search bar should support system Highlight colours as well.

Differential Revision: https://phabricator.services.mozilla.com/D61461

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

browser/themes/shared/compacttheme.inc.css

@@ -9,5 +9,4 @@
 :root:-moz-lwtheme-brighttext {
   /* !important to override LightweightThemeConsumer.jsm */
   --autocomplete-popup-background: #2A2A2E !important;
-  --autocomplete-popup-highlight-background: #0060DF;
 }