Source

File: wp-includes/class.wp-styles.php

public function in_default_dir( $src ) {
		if ( ! $this->default_dirs ) {
			return true;
		}

		foreach ( (array) $this->default_dirs as $test ) {
			if ( 0 === strpos( $src, $test ) ) {
				return true;
			}
		}
		return false;
	}