White background when loading the website

Hello,
I have a website with a black background.
With activated plugin “Cookies and Content Security Policy”, white background is loaded first, after that my set black background comes.
When I deactivate the plugin “Cookies and Content Security Policy”, my real black background is immediately loaded.
Do I have to set something special in the settings “Cookies and Content Security Policy”?

The page I need help with: [log in to see the link]