We use "is-in-chrome-docshell" rather than "is a chrome doc" so that about:
pages that are loaded in the content area (like about:addons etc) follow the
general content theme as well.
Cache the relevant color schemes since having that many branches on the default
system theme made me a bit uncomfortable, and this code can be called quite a
lot... Though it probably isn't such a huge deal. This makes us initialize the
theme right away, so make sure subclasses do the right thing. This in practice
was only an issue for GTK, which does cache the "system theme is dark" bit.
Differential Revision: https://phabricator.services.mozilla.com/D128674
