Comment Plus customization and theme files

This is for site . I am using Comments Plus plugin. You can see what I have so far here:

I am going to attempt to create a theme that resembles this page but I am not sure how exactly (in case you have an tips with CSS for the talk bubble and spacing)

What I want to make sure I am doing correctly is playing with CSS on the right file. If I understand correctly I would move this file lib/forms/wdcp-custom_comments_template.php to my themes folder. My theme is from studiopress and uses Genesis and Lifestyle as the child. So, I think I would move it to... Genesis would be my best guess.

I have spent 2 days on just trying to have my comments look like how I imagine--pretty basic, but I DO want to stick with WP as my base. When I tried Disqus, etc I had a lot of negative feedback. Many of my commenters comment at least one time per day, some 5 times, so I need it to be clear to them.

Thank you in advance!