The newtab page is styled according to a `lwt-newtab-brighttext`
attribute, which until now is only present in the newtab page, and not
exposed to the chrome. This is a problem for elements like feature
callouts, which we want to fit in with the newtab content, even though
they exist in the chrome. That's okay in many cases, because the
callouts also use theme properties, and there's usually a concordance
between `lwt-newtab-brighttext` and the content color scheme. But in a
few older (but very popular) themes, there are missing theme properties,
and it's possible for the newtab page to be very dark even though
prefers-color-scheme is not dark.
This patch resolves the issue by exposing `lwt-newtab-brighttext` to the
chrome, and using that attribute to theme newtab callouts.
Differential Revision: https://phabricator.services.mozilla.com/D208256
ca7244fbbc