Your code is correct, but it is not working because that specific 2nd level comment is an author comment which gets its own style
HTML Code:
ul.children div.bypostauthor {
margin:10px 0 0 30px;
}
I am not even sure if this ul.children div.bypostauthor is needed, will have to look through all the comments styling again. The first reply level (2nd overall comment level) used to be not indented at all to save horizontal space, in earlier theme versions but I changed this to have all sub level comments indented
Keep your style and reset the extra left margin for div.bypostauthor:
HTML Code:
ul.commentlist li ul.children li.depth-2 div.bypostauthor {
margin-left: 0 !important;
}