I saw in the statistics of this blog that someone arrived here with that exact search string. I thought that would only take me 20 minutes to quickly hack a simple web page that would spit game jam themes, so I just did it. I have a list where I compiled theme suggestions for the Ludum Dare editions 12 to 18 (I couldn’t find the information for the more recent ones). What this application does is just choose one at random and show it. You have a button to refresh, if you don’t like the choice. If you want to see the whole list, well, it’s not hard to find that in the source of the page.
Edit Nov 3rd 2013: I made a new version with the collected theme suggestions for the last year and a half at the Berlin Mini Jam.