Tip: if you want to change the default theme in a #WordPress Multisite network, use: define( ‘WP_DEFAULT_THEME’, ‘themename’ );
— Konstantin Kovshenin (@kovshenin) March 16, 2012
It’s important for a network (WordPress Multisite) to have a valid default theme for newly created sites. The current version sets this to twentyeleven in default-constants.php. If your default theme isn’t installed or disabled network-wide, you might face some errors when creating new sites in the network. You can define
WP_DEFAULT_THEME in your wp-config.php file.