main: Add support for -high-contrast theme variants

While the default Shell style is fairly decent with regard to
accessibility requirements, having the ability to tweak certain
aspects where the regular style works less well is still useful.
For this purpose, try to load a -high-contrast theme variant of
the default stylesheet when a high-contrast theme is requested
(as determined by the GTK+ theme name).

https://bugzilla.gnome.org/show_bug.cgi?id=740447