Yay! Congrats on your new Kindle. I've only used the font hack (which I love), so have no firsthand knowledge of the screensaver hack. But, I did find instructions here:
http://www.kindleboards.com/index.php?topic=11566.0Are the hacks safe? I think so. I haven't read about anyone having any real, non-fixable, problems with them. I'm sure others will chime in. There is one known scary thing that can happen (at least with the font hack, I'm not sure about the screensaver hack) where your Kindle goes into a loop if you don't follow the instructions. I'm kind of embarrassed to admit this, but it happened to me because I got impatient and slid the slider button while the hack was installing even though the instructions very specifically said not to do that.

I was worried that I'd bricked my Kindle, but I remained mostly calm and followed the instructions on how to fix things should this happen and everything turned out fine.
If you don't see a skin you like, you can make your own custom skin with Decalgirl. Here's the place on their website to go to if you're interested:
http://www.decalgirl.com/page.view/customwork.htm