Follow the steps mentioned next to change the default blog theme:
Download the New Blog Defaults plugin from http://wpmudev.org/project/New-Blog-Defaults.
Place the
cets_blog_defaults.php
file in your/wp-content/plugins
folder.Activate the plugin by opening the Plugins section of the WordPress MU admin panel.
Now go to Site Admin | New Blog Defaults. You should see a page with a lot of options.
For now, the option we are interested in is Default Theme, which is at the bottom of the page. Select the theme Blue User and click Save Changes.
Congratulations, you've just installed your first plugin! This plugin allows you to set some basic default options to ensure that everyone who creates a blog on your site gets exactly the right look and feel. You can find plugins that will allow you to do almost anything you can imagine with WordPress MU, so be prepared to add many more plugins as your site evolves.