public static function generate_css() {
		// Include core styles so we can eliminate another stylesheet.
		$core_css = file_get_contents( Popup_Maker::$DIR . 'assets/css/pum-site' . (is_rtl() ? '-rtl' : '') . self::$suffix . '.css' );

		/**
		 *  0 Core
		 *  1 Popup Themes
		 *  5 Extensions
		 * 10 Per Popup CSS
		 */
		$css = array(
			'imports' => array(
				'content'  => self::generate_font_imports(),
				'priority' => - 1,
			),
			'core'    => array(
				'content'  => $core_css,
				'priority' => 0,
			),
			'themes'  => array(
				'content'  => self::generate_popup_theme_styles(),
				'priority' => 1,
			),
			'popups'  => array(
				'content'  => self::generate_popup_styles(),
				'priority' => 15,
			),
			'custom'  => array(
				'content'  => self::custom_css(),
				'priority' => 20,
			),
		);

		$css = apply_filters( 'pum_generated_css', $css );

		foreach ( $css as $key => $code ) {
			$css[ $key ] = wp_parse_args( $code, array(
				'content'  => '',
				'priority' => 10,
			) );
		}

		uasort( $css, array( 'PUM_Helpers', 'sort_by_priority' ) );

		$css_code = '';
		foreach ( $css as $key => $code ) {
			if ( ! empty( $code['content'] ) ) {
				$css_code .= $code['content'] . "\n\n";
			}
		}

		return $css_code;
	}