HighContrast fixes

[Matthias Clasen <matthias clasen gmail com>]

Hi,

while Lapo has done heroic work in the last few weeks to put Adwaita
back together (and nicer than before!), we have somewhat neglected to
keep the HighContrast theme up with the GTK+ CSS changes. Lapo and
Jakub are now looking at fixing it up quickly; but we're probably not
going to ask for individual freeze break requests for that - I hope
that is ok.

Matthias