Customizing comments?

So, I'm probably retarded, but I've spent all of the last two hours searching for information on how I can customize the look of the comments of a post. Basically, I want each comment to show up with a white background, and rounded black border, just like I styled my posts and side widget bars to have. Is this possible, or am I missing something?

Also, as of right now, these weird gray lines show up randomly on the page when you view the comments under a post. I would show you, but my site is running locally at the moment.

Any ideas? I appreciate the help! <3