In the related CSS files find comments.css
and look for

#comment_preview {
	background-color: #eee;
}

Change that to

#comment_preview {
	background-color: #000;
}

Where would I find this CSS file? As the only one I see is in the tm-dark folder, which is the skin that I am using, though changed to my liking. And that CSS file doesn't have that code. I tried adding it but it didn't change anything.

Your CSS file for that skin is located
http://www.gararion.com/skins/tm-dark/style.css
The comments CSS file is located:
http://www.gararion.com/rsc/css/comments.css

Ah, thanks. I knew where the css for for the template was, didn't know where the other css was. Got it all figured out now :)