Refactor dark theme detection into a function
Fixes #133 (closed).
We need to instance a Gtk.Widget to be able to use
Thus, we use the singleton pattern to avoid allocating an object every
time we call
I'm not 100% sure about the solution. I mean, I believe we do need a
Gtk.Widget instance so there's no way around placing this code inside a
Gtk.Widget subclass, but perhaps there is a better place to put this or a better solution instead of a singleton.