Okay I think you need some general background knowledge more than a specific answer. Here is a tutorial that should explain the basics of child themes to you, and how you can work with the CSS, recursively search to find certain parts of the code, etc:
http://xcartguru.com/tutorials/wordpress-tutorials/building-a-custom-wordpress-child-theme/
If you watch that fully and pay attention, looking at your file structure of your site for refernce, then you should be able to divine for yourself where the change is needed.