Yes, the interface can be re-themed, and even re-arranged and completely redone. Start searching for HADesigner (its the UI dev program) here and in the wiki. However, it is very crude and not very user friendly, and you will probably need a little programming experience to understand and use it. Even more, most of us are just learning about it ourselves, so finding information on it isn't always easy.
Buttons can be added to the categories (Lighting, Media, Climate, etc..) through the web admin very easily without using HADesigner. The buttons that you add this way are called Scenarios. There are also some other simple customizations that are easily done via the web admin (floorplans, background images..)