So there are a bunch of CSS files. I can't figure out which one to edit to change the look of a standard item listing template. For every change I make, nothing appears to change on the site. No matter if I have the option check to use the K2 Base CSS or not, nothing works. I even deleted all of the CSS files from everywhere I could find, and the page still didn't change.

How the heck do you change them?

If you're working in the templates section inside the Joomla admin panel, you won't find them; look in your file structure and open the appropriate style.css in whatever editor you want to use. Look in the k2 folders in components (for main content) or modules (for the specific modules).

The Firebug extension for Firefox is a huge time-saver here. Inspect the part of the page you want to edit to get the name of the div you need to find. The file names will all be "style.css" (so far as I've explored), so while you still have to figure out which style.css to edit, you'll at least know that if it isn't "style.css," it isn't a K2 styling.

Just create your own! This way you can keep your changes even by an update. To make your css statement overrule the css of K2 itself use the following:

.class {
color: #ffffff !important;
}