Where do you want to make changing on background color?

If whole page, go to Dashboard > Course structure > Project settings > Custom CSS/LESS code  and add this code:
body { background-color: #fff; } 

If you want to change background color of a block, then go to Dashboard > Course structure > Page structure > Block settings and from Settings > Classes add a custom class, for example: cBlock-05. Then go to Dashboard > Course structure > Project settings > Custom CSS/LESS code and add this code:
.cBlock-05 { background-color: #fff; } 

Remember to replace #fff to your custom color.