I did this myself a couple days ago... I deleted all themes except the custom theme I was using. But oops, I found that I need at least ONE in order for my custom theme to work, so I kept the BB Dimension Zen one.
With only two themes on my 8800, I now have 31.6 meg free after a battery pull (previously 25.0 meg free, so I gained 6.6 meg just by removing themes).
Supervising Manager of the Department of Redundancy Department.
BlackBerry Certified Support Specialist