Our GUIs should support themes (or "skins"). That is, materials and colors should be easily switchable, possibly at runtime.
This feature should be easy to implement: We only need to group all materials, textures and color definitions for a theme in a common subfolder of the GUI -- the contents of the subfolder defines the theme contents, the name of the subfolder is the name of the theme.
The only truly new extra feature that seems to be required is for color definitions, for which we have to introduce e.g. "color tables" as a means of indirection.