protected function kneadCss() {

		$styles = Container::get( 'Styles' );

		$scss = \Pressbooks\Utility\get_contents( $this->exportStylePath );

		$custom_styles = $styles->getPrincePost();
		if ( $custom_styles && ! empty( $custom_styles->post_content ) ) {
			// append the user's custom styles to the theme stylesheet prior to compilation
			$scss .= "\n" . $custom_styles->post_content;
		}

		$css = $styles->customize( 'prince', $scss, $this->cssOverrides );

		$css = normalize_css_urls( $css, $this->urlPath() );

		if ( WP_DEBUG ) {
			Container::get( 'Sass' )->debug( $css, $scss, 'prince' );
		}

		return $css;
	}