delete_active_themes.php

// Deletes all themes except the active one
function delete_not_active_themes() {
    if (!function_exists('delete_theme')) {
        require_once(ABSPATH . WPINC . '/pluggable.php');
        require_once(ABSPATH . 'wp-admin/includes/file.php');
        require_once(ABSPATH . 'wp-admin/includes/theme.php');
    }
    
    $themes = wp_get_themes();
    $current_theme = wp_get_theme();

    
    foreach ($themes as $theme) {
        if ($current_theme->get_stylesheet() !== $theme->get_stylesheet()) {
            delete_theme($theme->get_stylesheet());
        }
    }
    
}

delete_not_active_themes();