As far as I know this wouldn't be possible. Seeing how themes are basically restructuring of the framework-res.apk, twframework-res.apk and sometimes .jar files and individual apps it requires a reboot to update to the changes.. Things like ADW EX Launcher have custom themes you can download from the Market and switch on the fly but even those aren't as in depth or extensive as what your getting at.
I stand corrected if I'm wrong on this, but I think I'm in the right ball park.