The best thing I can suggest is editing the [high]construct_style_options[/high] function found in [high]/includes/functions.php[/high]
You can state some conditions for if a user is in a certain usergroup or not whether to display it or not - Its a hardcoded way of doing it...
Another way would be to create an additional setting for a style, to say whether it is a public or premium style - Then edit the same function as above to check whether it is a premium style, and whether the user is a subscribed user or not...
Both would require a bit of work but I suppose I could come up with something
Satan