Here’s a quick tip that’s very easy to implement. When new sites are created with WordPress Multisite, the default behavior is to assign the WordPress TwentyTen Theme.
However, if you’d like to use a different theme, you can add the WP_DEFAULT_THEME constant definition to wp-config.php file, like so:
Replace ‘classic’ in the example above with the theme name that you want to define as the default.
Setting a default theme is very important, if you intend to remove the TwentyTen theme completely, according to the codex: