Comment 11 for bug 573118

The solution I found to this problem was to upgrade to 0.23 first, and then on to 0.23.1

If you have already gone straight to 0.23.1, then add the 0.23 repos above the 0.23.1 ones in your sources.list, and do apt-get update / upgrade

The 0.23 theme packages seem to be missing from the 0.23.1 repos, so they never get updated beyond 0.22