Not easy. What colour do you want it to be?
There’s only white space at the bottom, BTW, because your content doesn’t reach the bottom of the viewport. There’s a very small (20px) bit of white that shows through right at the very bottom (when I view it on my mobile phone in landscape) because there’s a 20px bottom margin on the #page
div, but it’s easy to get rid of that (preferably in a child theme stylesheet – I’ve already spent time trying to help somebody out tonight whose customisations were overwritten by a theme upgrade).
However, if you want the white background not to show no matter what the height of the #page, then it’ll require a bit of work. Mainly, it’ll require setting the body background to whatever colour you *do* want to see below the #page
and then wrapping the #page
in another div to which we can give the white background. Or you could use a background image…
Cheers
PAE