the colorswatch used to be made of two box the same size laid one on top of the other and additional gtkimage on top of it for the check graphics now it's messed up the "overlay" sims to act on the check box istead of the layer if should act on, try this out in the inspector:

colorswatch { border: 1px solid black; }
colorswatch overlay { border: 1px solid red; }

Note the double border on the colorswatch which I guess it's from
the old '.overlay' and the colorswatch itself.
Consider that with the current status of gtk css the gtkimage for the check graphic is not needed anymore since I can use the overlay background directly for it.