Here are three links to download free themes for Sony Ericsson UIQ 3 based M600i:

1) http://gallery.mobile9.com/c/sony-ericsson-m600m600i-themes/1/
2) http://wap.sonyericsson.com/fun/mxhtml/home?cc=GB&dn=M600i&lc=en —> Click on Themes
3) http://www.se-uiq.com/themes-m600-eng.htm

Update:
Official themes from UIQ Technology