Category Background pics but not at the lower levels posts page

I can't seem to find the setting where I can set a background picture for a category at the headings page but then revert back to no background at the lower level posts page for that same category.

Noting that this very forum does exactly what I am wanting to do too, it must be available somewhere.

Here's my example

I am happy with this background at the category heading level:

but this looks terrible and is automatically being brought through on the Posts detail page:

I would prefer that at the detail posts it is simply a blank background, but would retain the background image at the upper level.

As I said that is exactly what this WixForum is configured to be like, so I am hopefully just missing a configuration somewhere