In the Admin menu, choose "Edit or Delete a Forum". Choose that sponsor's forum. Directly above the table for configuring which groups have read/write access to the forum, there is a dropdown box that says, "What stylesheet do you want to use for this forum?". You can specify a different stylesheet for each forum.
However,

A user's stylesheet overrides this. It only works if the user's stylesheet is set to "Always use default" in their control panel. If it's set to "Always use default", it changes to what you've designated as the default for that forum. Obviously you could just take away the option of choosing their own stylesheet and just set them all to "Always use default."