Hi There,
Thank you for writing in! Please navigate to Appearance > Customize. On Customizer under the Layout and Design set a random image for Background Pattern and when the Background Pattern shows on the preview, just remove it. This is to remove the default Background Pattern (gray) that we currently seeing now.
Then just above that background pattern option, set the Background Color that you want.
Bye the way, I can’t access your site at the moment.
Hope it helps, Cheers!