Set default theme for subsites

1) http://wpmututorials.com/how-to/change-the-default-theme-for-sites-without-a-plugin/

Basically, something like this on your config file:
define(‘WP_DEFAULT_THEME’, ‘classic’);

where ‘classic’ is replaced with the folder name of your theme.

2) Whatever theme you want to use on your main site, do not network activate. Just apply it manually via Network Admin -> Sites -> Edit Sites -> Theme 🙂