Print GTK+ theme name if --debug is enabled

Should help debug issues like #29 because people often don't know what
the theme is called.
Invert the icon contrast selection logic

Because it was wrong: it picked a bright icon for bright backgrounds and
a dark icon for dark backgrounds.

Add manual overrides because the corrected logic gets it wrong for the
Adwaita, Ambiance, Radiance.

Fixes #29.