Any theme that has not been activated for the entire site can be activated on a blog as follows:
1. Locate the blog you want to enable specific themes on by either:
a) Searching for the specific blog by clicking on Admin
b) Clicking on Blogs
2. Once you have located the blog click on Edit (hovering your mouse over the domain name brings up the action menu)
c) Select the themes you want to enable and then click Update Options.
WARNING: Never edit any information in the left hand column in the Edit Blog area!
See Next: Editing your site options