function ‘theme_settings_page’ not found

I want to add change apearance setting to my theme and I am using this code:

//admin pannel function theme_settings_init(){     register_setting( 'theme_settings', 'theme_settings' ); } //افزودن تنظیمات به منوی پیشخوان function add_settings_page() {     add_menu_page( __( 'تنظیمات'  ), __( 'تنظیمات'  ), 'manage_options', 'settings', 'theme_settings_page'); }  add_action( 'admin_init', 'theme_settings_init' ); add_action( 'admin_menu', 'add_settings_page' );  

and there is an error: ” Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘theme_settings_page’ not found or invalid function name “

can anyone help me to fix it?