Tips on Customizing Custom CSS

How to Hide the "Custom Code" Admin Menu

function plt_hide_custom_css_editor_menus() {
	//Hide the "Appearance → Custom Code" menu.
	remove_submenu_page('themes.php', 'edit.php?post_type=ff-customcode-item');
}

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

Where do I put this code?

How to Hide Custom CSS Meta Boxes

function plt_hide_custom_css_editor_metaboxes() {
	$screen = get_current_screen();
	if ( !$screen ) {
		return;
	}

	//Hide the "Custom Code Content" meta box.
	remove_meta_box('CustomCodeContent', $screen->id, 'normal');
	//Hide the "Custom Code Logic" meta box.
	remove_meta_box('CustomCodeLogic', $screen->id, 'normal');
	//Hide the "Custom Code Type" meta box.
	remove_meta_box('CustomCodeType', $screen->id, 'side');
	//Hide the "Custom Code Placement" meta box.
	remove_meta_box('CustomCodePlacement', $screen->id, 'side');
}

add_action('add_meta_boxes', 'plt_hide_custom_css_editor_metaboxes', 20);