Currently, we do a check in our rust code to verify when document colors are used [https://searchfox.org/mozilla-central/rev/7ed8e2d3d1d7a1464ba42763a33fd2e60efcaedc/servo/components/style/gecko/media_queries.rs#273]. This check takes into account mIsChrome and mUseAccessibilityTheme.

On the C++ side, we do the same check in our backplate code: [https://phabricator.services.mozilla.com/D42979#inline-279931] -- using this link for now, but will update with a searchfox permalink once this lands.

It'd be nice to consolidate this check into browser_display_document_color_use, so we don't have to duplicate the logic on both sides.