widget: Fix theming

The application specific theming was removed from the gtk builtin theme.
Unfortunately there's *still* no way for a library to just drop a
css file in a ressource somewhere that will be automatically picked up
by gtk, so just add to the existing workaround with the explicit css provider.

https://bugzilla.gnome.org/show_bug.cgi?id=735081
Updated Traditional Chinese translation(Hong Kong and Taiwan)