Remove use of :-moz-lwtheme-{bright,dark}

This pseudo-class won't be supported in Firefox 100,
and we can just use :root[lwthemetextcolor="bright"] instead.

chrome/dark_theme_aware_statuspanel.css

@@ -6,7 +6,7 @@ Makes statuspanel aware of the the theme text color
 Meaning, if theme text is light then the statuspanel uses dark background and light text. On themes with dark text the statuspanel has normal appearance.
 */
 
-#statuspanel-label:-moz-lwtheme-brighttext{
+:root[lwthemetextcolor="bright"] #statuspanel-label{
   background-color: rgb(50,50,52) !important;
   color: rgb(187,187,189) !important;
   border-color: grey !important;