Add explicit colors to labels, buttons and entries. Make colors role based

The Pond GUI currently is almost unusable if you have changed the theme to different colors. (For example the Gnome 3 dark theme). As a first step to fixing this, these commits refactor the GUI to use role based colors only, and explicitly setting the colors in all places where necessary.

There are still some issues. Specifically, the main background of the UI, the background of buttons and entries doesn't seem to honor the widgetBase background specification at all - not sure why this happens.