Can't edit child comment borders

I have the theme Atahualpa 3.6.1 and for all I know this could be a theme related issue. but since i couldn’t change child comment borders in the theme options, I am guessing it is a larger, wordpress, problem.

See what I mean here

At first all of the child comments were grey dotted, but then on the atahualpa forum I was advised to add:

ul.children div.bypostauthor {
	border-style:solid;
}

to the css inserts.

The problem is, I dont want just the author’s child comments to have the correct border which div.bypostauthor is doing, but I dont know the correct code to change this. Thanks!