I don't know what skin you are using, but you can edit all the related css files to your hearts content.
The main css file is usually style.css
<link rel="stylesheet" href="style.css" type="text/css" />
and it loads all related css
Where you find them is usually as follows, which you will find at the top of your skin's style.css
@import url("../../rsc/css/basic.css"); /* Import basic styles */
@import url("../../rsc/css/img.css"); /* Import standard image styles */
@import url("../../rsc/css/blog_elements.css"); /* Import standard blog elements styles */
@import url("../../rsc/css/forms.css"); /* Import default form styles */
@import url("../../rsc/css/comments.css"); /* Import default comment styles */

I am editing the style.css thats in my skin folder. The problem is I can't make my own CSS classes. It ignores them for some reason. I can edit the CSS classes that were originally in the skin. I am using the custom skin that comes with b2evolution, I am just tweaking it.

You have to call your new css class from within your files.
ie
<div class="bItemRow">blah blah</div>
<div class="bItemRow2">blah blah</div>