GUI theming is made at two levels in glob2: widget theming and background theming
Widget drawing is done by the Style helper object, declared in libgag/include/GUIStyle.h. It has a default implementation in libgag/src/GUIStyle.cpp. Glob2 reimplements it partially in src/Glob2Style.h/.cpp to have things like bitmapped buttons, etc...
Each GUI screen inherits from the Glob2Screen class, defined in src/Glob2Screen.h and implemented in src/Glob2Screen.cpp. This class reimplement the Screen::paint virtual method so that a custom paint method is implemented.