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You can use the theme changer module. It's kinda tricky... I had to rename my custom theme "default" and then set the theme included with the module as the default in the admin area.... Then it works.
Then you can assign modules different themes and even custom meta tags and page titles. You can also clone your default a few times, rename it, assign each module it's own version, and then have different javascripts, style sheets and stuff by editing the theme.html file for each module's theme.
Magick can never be restrained, but when freely given is thrice regained!