TOPIC:

Not sure if this is a bug or a feature! Using Xerte 3.11.

The header titles colour changes OK.

The background colour changes OK in the other themes I've tried.

It's neither.
With some of the older themes they have a gradient applied which isn't being changed by a simple background:xyz;
As far as I can see with a quick check those themes are default, flat white, simple black & white; xhibit base theme - typically the old themes with images as buttons in the footer rather than FA Icons.
If you want to use one of these themes and change the header background colour via the new optional property try adding the following to the css for that page:
#x_headerBlock {
background: none;
}
HTH
Ron

That works! As does:

#x_headerBlock {
background: black;
}

(for the pages I don't want to change)

Many thanks.