Fix GTK theme errors

[taprobane99]

Selecting Breeze as the GTK theme shows the SBE buttons on apps like Firefox. But the size, spacing, and hover effects don't match the Qt theme. On a 1080p monitor the buttons are tiny, although on a 4K monitor they are correctly sized. There is also a strange bug on a 1080p monitor in which the maximise Gtk button enlarges on hover, but the other two don't. This is also present in standard Breeze Gtk.

Perhaps SBE should have its own GTK theme to fix these problems?

[kupiqu]

Please note SBE is currently unmaintained. Pull requests are welcome