Would using the background color of the theme itself work? Here is an example as a starting point. This will remove the dark coloring from the comment form in the Motion theme:
#commentform {
position: relative;
}
.highlander-dark #respond #comment-form-comment, .highlander-dark #comment-form-share-text-padder {
background: none !important;
border: none !important;
box-shadow: none !important;
}
.highlander-dark #respond .comment-form-service {
background: none !important;
}
.highlander-dark #respond #comment-form-identity {
background: none !important;
border-top: none !important;
}
#comments #respond p.form-submit input {
background: white;
}
This will remove the dark coloring from the comment form in the Motion theme and also move the login buttons below the "Post Comment" button, but I would caution against doing it this way because when you use absolute positioning it can cause funky behavior for different browsers if you're not careful and it also may break if the login form layout is changed at any time such as if new options were to be added. If new options were added and it made the login section longer, then you would need to adjust the height and top declarations in this set of CSS:
#commentform {
height: 400px;
position: relative;
}
.highlander-dark #respond #comment-form-comment, .highlander-dark #comment-form-share-text-padder {
background: none !important;
border: none !important;
box-shadow: none !important;
}
.highlander-dark #respond .comment-form-service {
background: none !important;
}
.highlander-dark #respond #comment-form-identity {
background: none !important;
border-top: none !important;
position: absolute;
top: 230px;
width: 640px;
}
#comments #respond p.form-submit input {
background: white;
}