Changing page background

I want to change the background color of the page from white to a different color but can’t see where to do it. The body tag is not changing it.

Also I want the user to pick their colour theme for the app. Is this possible to do. What would be the best way? Would I make a variable in SASS and use jquery or something to change the value of it?

Found in _variables.scss
$base-background-color: #fff !default;