As per Andy’s response above about needing to look at things more closely, the same applies, but here is another quick fix to get you up and running.
You have two options here, so once you’ve decided make sure you only use ONE of the two methods..
As the text is already white, the easiest thing to do would be to make the background black by adding:
#spMainContainer .spThanksList {
background: #000000 !important;
}
However, if you wanted the white background so the text stands out, you can add:
#spMainContainer .spThanksList p,
#spMainContainer .spThanksList span,
#spMainContainer .spThanksList a:link {
color: #000000;
}
And of course with this second method, you can break these rules down individually to give the links a different colour if you so wish.
Remember, only use one, and you can add this to the end of the unified.php stylesheet in your custom or child theme, before the PHP at the end.
Hope this helps for now!