Commentlist: bypostauthor problem with children list

Read the comments in your linked forum post on wordpress.com. It’s just CSS. You are styling the elements based on the li.bypostauthor list element. Therefore every other element which is a child of this one will also receive these stylings. The ul.children list is inside the parent li list element. Look at your sourcecode, there you have your answer:

<li class="bypostauthor">
    <a href="#">SUB-COMMENT</a>
    <ul class="children">
        <li><a href="#">SUB-SUB-COMMENT #1</a></li>
        <li><a href="#">SUB-SUB-COMMENT #2</a></li>
    </ul>
</li>

You could change this behavior with > in your CSS or overwrite the styling for all children inside the .bypostauthor list element. E.g.

CSS

li.bypostauthor a {
    color: red;
}

li.bypostauthor li a {
    color: blue;
}

Or with >:

li.bypostauthor > a {
    color: red;
}

As described by gwideman in the forum:

1. xxx yyy { } means select any yyy somewhere within an xxx.
2. xxx > yyy { } means select a yyy which is an immediate child of an xxx.