Since HighContrast is now a variant of Adwaita, we can get HC and HCI
support for free by just importing the SCSS files with its color
overrides.

However, it must be imported after `colors`, but before `drawing`, and
it's not possible to do conditional imports, so move the `colors` import
to the individual theme styles from `definitions`.

Introduce $high_contrast variable, it will be used to add some HC-specific
overrides later.

Fixes #221