The comment form is has about a billion selectors that control the look of it so you will just have to play with the stuff below (which may not be all of it) and figure out what each does and then go from there.
.highlander-dark #respond form {
background: none repeat scroll 0 0 rgba(255, 255, 255, 0.3);
}
.highlander-dark #respond #comment-form-identity {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0.8);
border-top: 1px solid rgba(0, 0, 0, 0.9);
}
.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-comment textarea, .highlander-dark #comment-form-share-text-padder textarea {
color: #EEEEEE;
}
.highlander-dark #respond .comment-form-fields div.comment-form-input.active {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0.37) !important;
border: 1px solid rgba(255, 255, 255, 0.25);
box-shadow: 1px 1px 2px rgba(0, 0, 0, 0.55) inset;
color: rgba(255, 255, 255, 0.8) !important;
}
.highlander-dark #respond .comment-textarea label {
color: rgba(255, 255, 255, 0.6);
}