Looks like whenever you get the file, it will just replace wp-layout.css
You’ll then have to alter any minor styling of elements to suit just as you would with any other wp style that you import.
It states that they are made with WP in mind, so I would assume it is all tagged correctly.
First though, backup your current css / index.php so that if it all goes wrong, you can quickly and easily revert to your current style.
And don’t forget their TOS.