Fix HighConstrast pathbar and floating statusbar.
HighContrast and HighContrastInverse have been poorly supported in this app for a long time because we ship many custom styles.
However, nowadays HighContrast and HighContrastInverse stylesheets are derived from Adwaita stylesheet, so the same custom styles can be applied cleanly. Let's do that.
Fixes #1614 (closed) and fixes #1469 (closed)