When rendered with metacity (e.g. with metacity-theme-viewer), the back background parts of HighContrast and HighContrastInverse actually show up transparent. This is because the corresponding rectangles were missing the filled attribute. In the HighContrast case, the gtk_arrow is getting drawn black on black (and there is currently no way to change the color), so we can as well draw it by hand to be able to change the color. The close button also deserves bigger width to be more visible. Fixes #211