This patches applied tweaks to the CSS rules that are meant to choose the right browserAction
theme icon based on the theme colors.
In addition to that this patch includes a tweak (unrelated to the changes applied to the CSS rules)
to the test that would be hitting a failure if the CSS rules are not applied to the browserAction
toolbar-button as the test cases from this test file expects.
This is currently split out from the parent patch including the rest of the frontend changes
because these rules seems worth an additional close look.
Differential Revision: https://phabricator.services.mozilla.com/D229993
3db97e7f16