public static function cache_css() {
		if ( false === self::$cache_dir ) {
			return;
		}
		$css_file = self::generate_cache_filename( 'pum-site-styles' ) . '.css';

		$css = "/**\n";
		$css .= " * Do not touch this file! This file created by the Popup Maker plugin using PHP\n";
		$css .= " * Last modified time: " . date( 'M d Y, h:i:s' ) . "\n";
		$css .= " */\n\n\n";
		$css .= self::generate_css();

		if ( ! self::cache_file( $css_file, $css ) ) {
			update_option( 'pum-has-cached-css', false );
		} else {
			update_option( 'pum-has-cached-css', strtotime( 'now' ) );
		}
	}