CSS for making asides background darker

So I like how when I comment to an article on my site my comment is ‘highlighted’ via a darker background than the other comments. Example:
https://fak3r.com/2006/12/16/velvet-underground-record-back-on-ebay/#comment-2426

Looking at the source when this happens I see:
<li id=”comment-2426″ class=”comment c3 byuser commentauthor-admin bypostauthor c-y2007 c-m01 c-d11 c-h10 alt”>

But I’ve been unable to understand which class is causing this. I want to know because I want to add this to my asides, which you can a few on my front page right now, so they have the same background, making it clearer that this is an aside, and not a regular article.

So, which class causes the background switch when I post to comments, and where would I add this for the asides to pick this up? I’m using K2 with the K2 Scheme set to spirit/sprint.css. Thanks -fak3r