I blame the Avatar. 😆
So I've just been messing with setting up a nodebb forum, and I noticed that once you add a background image to a category there isn't a way to go back to the standard flat color.
That would be a useful feature for those of us who like to fiddle around with the look and feel.
@LtKije See image.
I will add this threw me off too at first. You need to refresh the page after you upload the image and save to see the delete option.
Okay that explains it. Seeing it now.
An issue detailing this bug should probably be submitted on Github.