add switch_theme capability to contributor

Hi - first post and new to this great site - have learned a lot so far by lurking but need advice as to accomplish this task.

How can I tweak a user role to include a specific capability not inherently assigned to it? For example - I want the contributor level to be able to switch themes, would prefer to disallow the submenu(widget,theme-options, etc) if possible. Played with schema.php, menu.php but no dice - am I close?

Seems like there a many plugins and tips on issues very close to this - dsaders menus plugin, adminimize are close.

thanks

chris

ps - if you are wondering why, we are rolling out about 500 student blogs next week to student K-8 - assigning them as contributor but they will of course want a little creative license with their site. thanks for releasing batch create!