If a user chooses their own style, it will carry over with them for all the forums.
But if they choose "Default Style" it will use the one you've set up for them to use as default. Including the ones you've setup for them as the "Default Stylesheet" forum setting.
Basically, "Default Style" means, "The style to use when none is specifically chosen."
To update all your users to use the default forum style, run this command from Control Panel ┬╗ Database Tools | SQL Command
SET USER_STYLE = '0'
NOTE: If you are using a custom table prefix, you'll need to use that prefix, instead of "ubbt"