This is what my admin/customize/themes looks like.
You will note the default theme is missing as I deleted it from the admin area. I had the same problem with the default theme in that if I changed it to white, only admin would see the change.
white header bar where logo login search hamburger
rest of the page has a grey background
Firefox: unregistered (white background)
Even if I close all browsers on Laptop 1 and restart the machine, Firefox and Safari will load the site with a black background. So I presume there is a setting stored in the browser cache somewhere. But then I tested by clearing the Firefox history, clearing every history and data setting. When I loaded the site afterward it still loaded with a black background!
Similar to this but we have uploaded our logos and they appear in top left corner of the page when in admin view but not when in non-admin view? can anyone please help with this