Hi Emily,
For to change the back ground of the quiz , there are two option
1. You can go into Site Admin > Appearance > Theme > Basis (if you are using basis theme ) and change the main background color whatever you like
2. right click on the light blue region and inspect element find the correct id or class
and write it in custom css area in basis theme. like this
#idname{ background-color: green;}
.classname{ background-color: green;}
Regards
Rahul