Bug
1672097 - Part 1: Make Gtk theme overrides work with the RemoteLookAndFeel. r=spohl,karlt
When the RemoteLookAndFeel is not in use, content processes override the
Gtk theme in some circumstances (e.g. if the theme is a dark variant, we
try choosing the light one, since that works better with Web content).
When the RemoteLookAndFeel is in use, the parent process needs to handle
this. So we temporarily set the parent process to the overridden theme,
extract the FullLookAndFeel data, then restore the original theme. We
make sure to avoid throwing away any cached data and restyling every
document as a result of the temporary theme change.
Differential Revision: https://phabricator.services.mozilla.com/D98418