Enable a theme in all subsites

Enable a theme in all subsites.php

<?php

// RUN IT ONCE ONLY and USE as mu-plugin

/*
 * To use mu-plugins, go to /wp-content/ and find the folder with name 'mu-plugins'. 
 * If there is no folder in that name, then create a folder, name it 'mu-plugins',
 * create a file inside that, give any name you like and paste the code in there. 
 * You don't need to activate that plugin. Mu-plugins means must use plugins, so it will be activated automatically always. 
 * If you use mu-plugins then add a php start tag at the beginning of the code.
 */

$sites = wp_get_sites();
foreach( $sites as $site ) {
    switch_to_blog( $site['blog_id'] );
    // Change to your theme
    switch_theme( 'twentytwelve', 'twentytwelve' );
    restore_current_blog();
}