Another rounded graphic question

[shadowdancer36]

I've tried looking around but I can't seem to find the right posting to do this.

I'm trying to find out how to place rounded corners using images. I looked at the code for someone's forum that had them I saw the following:

.tcat_left {
background: url(http://www.theirsite.com/forum/images/misc/tcat_left.gif) no-repeat top left;
height: 36px;
line-height: 36px;
padding-left: 21px;
}

.tcat_right {
background: url(http://www.theirsite.com/forum/images/misc/tcat_right.gif) no-repeat top right;
height: 36px;
}

Question is, where do I place this code if I need to? I tried placing it in the extra css box with no luck.