Also tried the link on the options / colors page that says Background color for the body area can be set here: Body Background
The Body background is for the area outside of the page’s container.
Add this on Frontier Options -> CSS
article {
background-color: #000000;
color: #FFFFFF;
}
If you want, you can do the same for the widgets.
.frontier-widget {
background-color: #000000;
color: #FFFFFF;
}