Bug 1561552 - Use :root instead of #main-window in browser/themes/windows/compacttheme.css r=dao

Differential Revision: https://phabricator.services.mozilla.com/D36313

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

browser/themes/windows/compacttheme.css

@@ -17,10 +17,10 @@
   /* Prevent accent color overriding the window background for
    * light and dark theme on Aero Basic. This is copied from browser-aero.css. */
   @media (-moz-windows-default-theme) {
-    #main-window {
+    :root {
       background-color: rgb(185,209,234) !important;
     }
-    #main-window:-moz-window-inactive {
+    :root:-moz-window-inactive {
       background-color: rgb(215,228,242) !important;
     }
   }
@@ -66,7 +66,7 @@
 @media (-moz-os-version: windows-win7),
        (-moz-os-version: windows-win8) {
   @media (-moz-windows-compositor) {
-    #main-window {
+    :root {
       background: transparent !important;
     }
 
@@ -120,7 +120,7 @@
       color: InactiveCaptionText;
     }
 
-    #main-window[tabsintitlebar] #main-menubar > menu {
+    :root[tabsintitlebar] #main-menubar > menu {
       color: inherit;
     }
   }