Trying to change active theme on all sites in Multisite

Use switch_theme( 'theme_name' ) to switch a theme

Use wp_get_sites( array('network_id' => $wpdb->siteid) ) to retrieve all sites info including the blog_id

Loop through the retrieved blog ids and execute code within the scope of each blog using the switch_to_blog() function:

foreach( $blog_ids as $blog_id ){
switch_to_blog( $blog_id );
//Do stuff
restore_current_blog();
}