PlayStation Blog has posted a how-to guide on making themes for the PS3 XMB. Written by Noel Silvia, a graphic artist on the PSN team, the guide explains everything that users need. The required software for the task is an image editing application and extracting the files from HowToMakePS3Themes.zip.
We've yet to try making a custom theme ourselves, but we'd certainly like to know how people make out. For those considerably less ambitious, themes are also available by checking the PSN Store.