Multisite Default Theme
Allows to add a default theme option on your muiltisite. Everytime a new blog is created it will grab this option to set the default theme.
How to
Just grab the plugin and activate it (network wide) then go to settings > network settings and you'll find the option at the very end of the page.
Warning : this plugin does not change anything unless you create a new blog.
You could also use the WP constant WP_DEFAULT_THEME instead but in a multi-network installation you would not be able to set this per each network.