@Kroll

Yes, having skins for this pop up window is an option, but this could make the desktop heavy, particularly if you have other desktop applets active resulting in bad response times.. However if you notice, there is some amount of skinning, coloring achieved in the otherwise dry gtk calendar widget.

The pop up window inherits the color scheme of the "Day" font from desktop calendar widget. So if you set the "Day" font color matching the theme color of the Calendar.png on your Flip Calendar widget, the pop up calendar window will also inherit this color scheme ( look at the updated image in the first post)