How do I change to black background with white text?

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;
}

#wp-calendar th, #wp-calendar td {
    background-color: #000000;
}