It's not Ubuntu 8.1. It's Ubuntu 8.10. Version numbers are not decimal numbers. For Ubuntu, they choose to make their version numbers for releases the year and month of the release date. So 8.10 was released in October of 2008. The next release is due in April of 2009, and will therefore be 9.04.
You could try downloading the source and compiling it yourself. I've done this on Hardy (8.04) and Intrepid (8.10). I've never had difficulties, but then again I use the default theme.viewtopic.php?f=3&t=8636