How to Hide the "Playing Card Notations Settings" Admin Menu

function plt_hide_playing_card_notations_pcn_menus() {
	//Hide the "Appearance → Playing Card Notations Settings" menu.
	remove_submenu_page('themes.php', 'pcards-settings-menu');
}

add_action('admin_menu', 'plt_hide_playing_card_notations_pcn_menus', 11);

Where do I put this code?