Add regression test for theming capabilities

[provokateurin]

Only the static color is displayed as the dashboard background. I haven't looked into it at all, but I suspect a server bug as nobody touched this part of the code after it was implemented and working.

[provokateurin]

Indeed a server bug that is already fixed on master, but still present on 29 (and potentially other versions, but I didn't check).

[provokateurin]

Once fixed I will add a test to prevent regressions in the future.

[provokateurin]

nextcloud/server#46211