Frogboy: It was my understanding that they had an agreement with skinz.org when it was the sole storage area for Talisman themes. They also make mention of the sites they use on the 'add theme' section.
joetheblow: No, no built-in ability, but if you have PHP running on your machine, you can execute PHP files, either with a command, or at intervals with 'scheduled tasks' or some such. Theme files for most shells are just text files that can be parsed and altered by PHP scripts like any other file.
You can grab material off the internet and make menus and whatnot out of it, or you can use it to alert you to whatever by swapping out images, etc, like the recycle bin. On my fileserver I have a messageboard-type script, and I have PHP update messages to my desktop, along with the last entries from the ftp/web server logs, etc. Not much different than what Samurize or such apps do, but you aren't limited by using someone else's app.
You could do just as much with LS, or any shell that lets you refresh and pulls its theme info from a regular, parsable text file. I'm sure you could combine it with the scripting mods for LS as well.
I just wish refreshing was more low-key. Automated refreshing can get annoying when the whole thing kind of stutters for a second.