The first is the part that controls the comment area. The second the name, email and website fields. If all you are wanting to change is the background color of the input areas and leave everything else the same, then just keep and edit the "background" declarations and don't include the other declarations.
.highlander-dark #respond #comment-form-comment, .highlander-dark #comment-form-share-text-padder {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0.7);
border: 1px solid rgba(255, 255, 255, 0.13);
box-shadow: 2px 2px 3px rgba(0, 0, 0, 0.55) inset;
}
.highlander-dark #respond .comment-form-fields div.comment-form-input {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0.3) !important;
border: 1px solid rgba(255, 255, 255, 0.2);
box-shadow: 1px 1px 2px rgba(0, 0, 0, 0.55) inset;
color: rgba(255, 255, 255, 0.8);
}