How badly could I mess up my blog if I try to copy this css tweak?

When I was researching an issue I was having on the forum the other day, I came across this thread:
http://forum.bytesforall.com/showthread.php?t=7686

In the thread the person who posted it figures out how to rond the corners of their site. Here's how their site ended up looking:

http://codaforum.com/

I really like how this site is designed. I would have no idea it was Atahualpa if it wasn't on this forum! My site is pretty basic, and I'd like to make it a little more snazzy. Here's my link:

http://www.sam-mallery.com/

As I read through that thread, I'm just barely able to comprehend what Juggledad is suggesting to do. But, I like how this site looks, and I feel like I need to snaz up and take some chances.

Could I really screw up my blog if I started futzing around with the code in this thread?

Export your style and go to town, if you mess up too bad, you can always reimported your style and start again ( export/import available in veer 3.5.1)

Just looked into upgrading my version of Atahualpa, and I get this warning:

"Upgrading this theme will lose any customizations you have made"

I did tweak the CSS in the header a long time ago. Is this going to get lost?

You should always do a manual upgrade as described in this forum. That way if something doesn't go exactly as planned you can revert to your current version of Atahualpa. It's as simple as uploading the new version of Atahualpa via FTP making sure it is in a folder with a unique name such as atahualpa351 which is what the folder name in the zip file is anyway. The old Atahualpa folder will stay there for the time being. You have to move a few images and change a couple of paths but that is a small price to pay for the comfort of knowing you have a safety net.