I did make a pseudo-official TOG console consortium theme (because I was bored and wanted to know how to) - have a look at this post.
You can install a new theme downloaded on a PC by copying it to a USB stick if you want, I find that's the most convenient way. But if you're into browsing the 'net from your PS3, you can do that too. Just download the theme file (.p3t).
I know PureMongrel is using this theme on his super-mega-upsize-me-f*ck-off-huge home theatre PS3, and I have it as mine on my relatively-normal-size TVas well. I know a couple of the other TOG PS3ers downloaded it also.