Sort of. If you make a theme unavailable for users to choose, you can set a specific subforum to use a different theme and link to that, making it override the theme the user has globally.
That's all I can think of right now.
Nah, I need something that works globally.
I know the quick theme plugin from Ryan Gordon comes the closest to getting to this. But I need a way to actually link to the theme from an external page.
Well I might be wrong but I think a theme is just applied to templates - you can't just 'link' to a theme, you have to link to a page that is using that theme.
That's not true because you should be able to extract a URL from either the usercp form or quicktheme form, because it needs a URL to work. Otherwise, it would be a function that can be transformed into a theme.
My plugin simply updates the option in the users table from what they selected. For what you'd need to do is build a plugin that hooks into global.php and overrides the theme based on the id in the url if it's set.
Take a look at what my plugin does if you need something to start off of
I looked at your plugin, but I have no PHP knowledge.
I'll request a plugin for this. Thanks for your help.