Bug 1585971 - Fix chrome root element styling. r=bgrins

In bug 1583403, some styling was moved from a "window,dialog,wizard"
section to ":root" which changed the specificity and overrode styles
in common.inc.css. Changing common.inc.css to also use ":root" fixes
the issue.

Differential Revision: https://phabricator.services.mozilla.com/D48090

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

toolkit/themes/shared/in-content/common.inc.css

@@ -150,9 +150,7 @@
 }
 }
 
-html|html,
-xul|page,
-xul|window {
+:root {
   font: message-box;
   -moz-appearance: none;
   background-color: var(--in-content-page-background);