Don't hardcode colors

We use @theme_selected_bg_color for the blue 'stamps', so
it only makes sense to use @theme_selected_fg_color for
the text color.
Small optimization

GResource allows preprocessing for pixbufs, so lets
use it.
Look sharp in HighContrast too

The HighContrast theme is all about, well, high contrast.
Therefore, we should remove our fancy shadows and most other
theming.