Hello,
Unfortunately, currently these is no central way to edit the Background Colors at the moment. The Sections have different backgrounds as they are used in different ways in different demos. We recommend changing the background Color of the #wrapper instead which will change the Main Content's Background Color and then go ahead changing the Background Colors of other Sections as required. We are working on CSS Variables at the moment which you can use easily to control the Background Colors of different Sections. You can also let us know the Sections you would like to change in a Screenshot and we can provide you with the Selectors required to change.
Let us know if we can help you with anything else or if you find any further issues.