By default WordPress uses Twenty Fourteen as default theme for subsites in a multisite network. There many users who simply delete Twenty Fourteen theme and then rename the theme they want making default as Twenty Fourteen. This is an easy way but it has some limitations like the next update will wash all your theme files to new updated version of default WordPress theme. So here is a more easy and right way to set up default theme in WordPress multisite networks.
All you need to do is define WP_DEFAULT_THEME in your wp-config.php file. So open wp-config.php file present in the root directory of your website and then add a line like define('WP_DEFAULT_THEME', 'classic'); where ‘classic’ is replaced with the name of the folder of your WordPress theme. That’s all.