Changes

* A selected tab should use SelectedItemText / SelectedItem with a ButtonText border

Components which require a mapping that isn't explicitly described above should have their mapping approved by the Accessibility team.

Note: The goal of HCM (though named “High contrast mode”) is to provide a user interface which respects customisations users have made in their system preferences. Users may customise their palette to adapt their system for photosensitivity, colorblindness, or other vision needs that are not strictly helped by high contrast. For this reason, we do not simply duplicate the contrast requirements set in the first section of this doc.