Since your blog is brand new, I wouldn't worry about messing with it, no one is really going to see it.
When I mess with the CSS, I copy the original and do the change. Then I check it in the browsers. If it's okay, I move on, if not, I just paste the original back in. Works for me. Course, have a un-touched copy in case things get really messed up.
Just remember to check both FF and IE, they can and do differ, the buggers..
