The changes mentioned above have been applied to master. Feel free to test the stuff out and please give feedback if anything doesn't work out for you.
Wildcard sub-selectors have also been implemented, e.g.:
Window > * > Label { ... }
Matches to all Label widgets somewhere underneath a Window widget. This will later be overseeded by
Window Label { ... }
(like used in CSS). The > selector will still be available; it will act as the direct descendant, so
Window > Label
will match Label widgets that are direct children of Window widgets.