Remove Border

I am trying to remove the border between each post… I have narrowed it down to the following:

#category-posts-6410001-internal .cat-post-item {
border-bottom: 1px solid #ccc;
list-style: none;
list-style-type: none;
margin: 3px 0;
padding: 3px 0;
}

I just don’t know where that is located to go and edit out the border-bottom line of code.

If someone could point me to the correct file.. or explain how I figure out the correct file would much appreciated.