Changed background colour when in 'boxed' layout

I assume you are referring to the main background which is currently set to

body {
background: #dadada;
}

Not sure if there is a theme option to change it or not. If you don’t already have a Child_Themes or a custom.css you should install a plugin like custom CSS plugin then add the following to the custom css area.

body {
background: #000;
}

Perfect buddy cheers!