Changing Blog Post Comment Box Background Color

If you want the form background to be the exact same shade of green as the page background:

.squarespace-comments .new-comment-area .input form.comment-form textarea, .squarespace-comments .new-comment-area {
    background:#6b8374;
    color:#fff;
}

If you want the form to be a darker green to differentiate/make it easier to spot for viewers:

.squarespace-comments .new-comment-area .input form.comment-form textarea, .squarespace-comments .new-comment-area {
    background:#2b4a3f;
    color:#fff;
}

These change the text color to white as well for legibility.