Bug 1751088 - Choose tabpanel background based on content preferred color-scheme. r=dao, a=RyanVM

If the theme is dark but user prefers light pages, the background of the
tabpanel should arguably be light, since it can be seen across some
navigations.

Expose a -moz-content-prefers-color-scheme media query to chrome pages
so that our UI can correctly query it (and remove the unused -moz-proton
atom while at it).

Differential Revision: https://phabricator.services.mozilla.com/D136437