For matrix sorting questions, when you drag an answer into a corresponding box, there is a placeholder that appears to indicate you can release the mouse and drop the answer into the area. By default, this placeholder has a light blue background with a 2px dashed red border.
To change the background & border of this placeholder, you can use the following CSS:
body .wpProQuiz_content .wpProQuiz_placehold {
background-color: #fff !important;
border: 2px dashed #000 !important;
}
Change #000
and #fff
to any color you’d like.