Er well you go to the uh, little blue bar on the left side (XD) and you click on customize. Then you go to Custom Options and click on CSS. You paste the code there. And save your changes. =D
But there's one problem, I'm not sure if it'll work...

for I have never actually tried it...I've only read the instructions for some free LJ layouts. So it actually depends.
If it doesn't work don't eat me...*hides*