Hi Michal,
I agree with you regarding tools like Artisteer. My firm, i7MEDIA, specializes in mojoPortal and we do a lot of skinning. Send me an email and we can setup a call so maybe we can work on the skinning documentation together. There's a lot to cover and a lot of it is already covered. The biggest hurdle is just jumping in and trying some things. Once you do that, you'll see that for a seasoned web developer with a very good understanding of HTML, CSS and JavaScript, skinning mojoPortal is very straightforward.
You can contact me through the contact form on the i7MEDIA website.
Thanks,
Joe D.