custom (themed) window

[Gezim Hoxha <gezimetc shaw ca>]

Hi all.

I want to use gtkmm to make a window like this prototype (GIMPed): 
http://www.flickr.com/photos/15643075 N07/1682920763/ .

What's the best approach to doing this? Do I create some sort of custom
theme for my app, or create a complete custom window widget?

Anyway, I'm new to this gtk(mm) stuff, so any tips would be greatly
appreciated.

Thanks,
-Gezim