I have no idea why this is happening though.
If you want to use free themes, the new SnowFlat theme, included in 1.7 is very good. You might be able to get it to work in 1.6, I haven't tested that.
I am using the Snow Flat theme on the Q2A dev build. Although it was downloaded about a month ago, I have added all the latest commits from the dev tree on github to my code.
When I click Ask a Question, and choose a category, the processing symbol appears (AJAX?), and then the selected category disappears and has to be selected again. I have not modified the qa-page-ask.php file. Happens both on my localhost server and on the live one. Ideally, I need to fix this bug, anyone got any ideas? I will post here if I fix it myself.