I guess the comment I wrote isn't clear enough.  The CSS on this branch is indeed order-independent.  Placing this last is protection against future CSS that isn't fully specified.  Basically SceneModePicker was being given special treatment because it was alphabetically last in the list to use shared CSS, so it wasn't clear that the rules could someday get overridden by a new widget added to the end of the list re-importing the shared settings.

By placing the shared settings last, with the knowledge that the shared settings never intentionally overrule any of the individual widget-specific settings, you force all widgets equally to take steps to overrule the shared settings as needed.

Basically this is protection against making the same mistake again in the future.