After some Google Chrome updates my personal background pic on the google home screen where I have some shortcuts to sites disappeared.
It's gray background.
I'm using GTK theme to have the dark mode like on head bar and menus.
The only way to have a custom background is to have the classic chrome theme.
If I have GTK and try to upload an image it switches to classic in the chrome appearance settings which in turn switches title bar buttons.
With GTK theme (can not upload custom background image):
I had to try to replicate this. Since my home search is set to Ecosia and not Google, I lack a "Customize Chrome" button at the bottom right by default.
This definitely is a Google-Chrome Issue, though. Testing it the same way as you shows the same problem.
And I deeply dislike that you have to link to Google in order to access the feature.
I encourage you to report this bug to the Chrome Devs, as they already sorely desire to restrict GTK options, anyway. The Browser Developers all want full control over the appearance of the browser as they perceive it as their brand image.
So your report can benefit everyone by existing as pressure on the devs to not restrict user controls.
And it could be a simple and honest mistake, anyway.