static function getBaseTheme( $slug ) {
		$filename = static::getCustomCssFolder() . sanitize_file_name( $slug . '.css' );
		if ( ! file_exists( $filename ) ) {
			return false;
		}
		$theme = get_file_data(
			$filename, [
				'ThemeURI' => 'Theme URI',
			]
		);
		$theme_slug = str_replace( [ 'http://pressbooks.com/themes/', 'https://pressbooks.com/themes/' ], [ '', '' ], $theme['ThemeURI'] );

		return untrailingslashit( $theme_slug );
	}