Since we are likely going to see theme names like
Adwaita and HighContrast, make fallback work as follows:

Adwaita -> Default
Adwaita:dark -> Default:dark
HighContrast -> Default:hc
HighContrast:dark -> Default:hc-dark
HighContrastInverse -> Default:hc-dark

Other themes will fall back to Default, as before.