Multiple Themes per site I want to include a whole new theme into a site to run along side the existing site on new pages (which make up another 'site' on this installation)
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I have thought that I could just create new page templates prefixed with the new site name within the existing theme and it would all be seperate, but I didn't think about the stylesheets and scripts in the theme.info file.

I want to use completely new html, js, and css as the current stuff is horrible (I can just not render the current scripts and css, but I am wondering if there is a better way.). I also want to avoid unnecessary impact to the current installation as there are already 3 sites running on it.

Is this the best way to add another simple site to an existing Drupal installation?
The ThemeKey module may be able to accomplish what you're looking to do.

The granular control it gives you as to when to show a specif theme is very impressive.