different themes on wordpress

I’ve seen some sites powered by wordpress that have a different style or theme on different pages. I’m curious on how this can be done. Here is an example of what I’m talking about https://www.bartelme.at. You;ll notice if you click on his main post or main entry, or recent entry’s the main blue post part at the top goes away. I would love to know how this is done.

Anyone have an ideas?