It would be good to create a new permission for users/groups to be able to choose their own theme. A mod has recently been made - see accompanying thread (http://www.simplemachines.org/community/index.php?topic=165500.0) - but I think it would be a useful core feature.
FYI I originally requested this so that I could test out changed themes on a group of users (global mods in my case) before applying it to all - but without giving users the option to select their own theme.
This is a reference to a comment in the source code:
Themes/default/Profile.template.php, line 1703:
// Are they allowed to change their theme? // !!! Change to permission?
A permissions-based mod is created for this purpose:
http://custom.simplemachines.org/mods/index.php?mod=795