By the way for the window_pieces01 file I was able to fix the problem with the color chooser, but I wanted to keep the blue filled checkbox since I think it looked better with the ui. You can just change the outer border of the 2 boxes to white but keep the second box filled with the blue gradient. That way you get the best of both worlds, a blue checkbox and working windows. My 2 cents
By the way great UI, keep it up.