[Bug]: Background image for the setup page is not loaded if installed in a subdirectory (28)

[kesselb]

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Git checkout: e547247

Steps to reproduce

1. Try to install Nextcloud in a subdirectory (e.g. example.com/cloud)
2. Realize the background is missing 😞
3. See a failing xhr request for https://example.com/apps/theming/img/background/kamil-porembinski-clouds.jpg (initiated by core/css/guest.css)

Expected behavior

Background should be there ;)

[joshtrichards]

Same issue / same underlying cause in maintenance mode: #45164

Also heads up the image name changed in #46453

server/apps/theming/css/default.css

Line 110 in 5170c73

--image-background: url('/apps/theming/img/background/jenna-kim-the-globe.webp');