Reconsider highcontrast implementation
When theme selection (including dark mode) moves to libadwaita, we should reconsider how the highcontrast accessibility feature is implemented. Currently, it works by selecting the HighContrast theme (overriding the current theme). In practice, HighContrast is just a contrast-enhanced variant of Adwaita, and HighContrastInverse is just a dark, contrast-enhanced variant.
Instead of hardcoding HighContrast as theme name, I think it would make more sense to have a high-contrast-mode setting to go with a tentative dark-mode setting, and consider both of these in the theme selection algorithm.