The `browser.display.document_color_use` is an a11y feature to allow
users to override colors. This feature does not work for UA stylesheets
and results in the reported bug.
To resolve the bug, this patch changes browser_style to be added as an
AUTHOR_SHEET instead of AGENT_SHEET. To avoid backwards-compatibility
issues, the patch also wraps the stylesheet content in a @layer to
ensure that all rules have a lower precedence than any (unlayered)
extension stylesheet.
The `browser.display.document_color_use` pref is also controlled
through the `browserSettings.overrideDocumentColors` extension API.
For extra test coverage, this patch sets the pref with that API
instead of setting the prefs directly.
Differential Revision: https://phabricator.services.mozilla.com/D198117
3b4a42c169