It's easy. I was going to put up a vid to show you how to handle it, but my stupid computer at work just can't handle Camtasia real well. You could do this a few ways, all using the power of the ProBranchConstructor.
Now you could do like HB says and choose a random theme by using the image height. One problem with this. Wordpress will usually throw up a couple of paid themes on the page and if your random selection hits one of those, your template will fail because you are taken to a purchase page instead of your blog.
The other way is to have a list of the wordpress themes in a text file and randomly pull up one of the names out of there and use it. In this case you would have to use the OuterHtml element to find the theme. You would also have to make sure that you chose either match #0 or #1 because match #2 will just give you a preview. There is a problem with this method also. Since WP shows random themes on the page, there is no telling if the one that was chosen out of the text file is actually shown on the page. If not, the template will throw an error.
The best way is to find something that is on the page and will be there no matter what template shows and is not part of the paid templates. In this case if you look at the outertext element on all the free themes, you will find the element is Activate. The paid themes element is Purchase. So you would choose one of the activate buttons, right click on it and choose outertext as the attribute name and Activate as the attribute value. If you hit the search button you will find there are 24 matches on the page. Choose one of them. Choose Rize and then click and copy to template. When you get your template in the template editor, in the element properties box, use the random Macro from 0-25 and your template will choose a random theme from the page.
