If WordPress multisite is enabled, newly created subsites use the default theme that ships with the current WordPress version. In WordPress 4.0, that default theme is Twenty Fourteen. If the twentyfourteen theme is not available, the front end of a new subsite will appear blank until the site administrator manually enables another theme. In fact, WordPress multisite does allow you to define a default theme for new sites, and the setup is very simple. You only need to add one line to wp-config.php.
Set the default theme for new multisite subsites
In wp-config.php, add the following line before require_once(ABSPATH . 'wp-settings.php');. Replace your-default-theme with the directory name of the theme you want to use as the default.
define( 'WP_DEFAULT_THEME', 'your-default-theme' );After that, whenever a user registers a new subsite, WordPress will automatically use the custom default theme you specified.
PS: In my opinion, the user experience of WordPress multisite is not especially good. Blogging itself is not functionally complex, and if you expect the platform to grow large, it may be better to build the system yourself earlier rather than later.
