Go to the user profile → Go to Layout option → normally there would be four sub themes you can select from.
In previous options, there were no options for a particular user to select the theme, But in the new version (7.10). All the users can select the theme if admin allows them.
There are some css issues in this version but it works. I am working on theme css, Please visit me to check it out.
Be happy, Life is short 🙂