The Site Default option should be broken up from the current skin chosen in the preferences since people like being able to force what skin they are using and don't want it randomly changing (should a site change their default).

For example, It could be something like:
( ) Site Default (Currently: xxxx | Preview | Custom CSS | Custom JS)
(*) MonoBook (Preview | Custom CSS | Custom JS)
( ) Simple (Preview | Custom CSS | Custom JS)
( ) Vector (Preview | Custom CSS | Custom JS)

Version: unspecified
Severity: enhancement