that would be the browser displaying its UI background color.
Some browser support
theme-color meta that allows you to set the UI color.
1. Create a new Hook Element in Appearance > Elements
2. Add this meta tag to the hook text area, and change the content hex color to what you want:
<meta name="theme-color" content="#000000" />
3. Set the hook to
4. Set the Display Rules to
5. Publish that element.
Clear the caches on your device, and let me know.