[Bug]: `enforce_theme=default` Disables Browser-Based Dark/Light Mode Detection

⚠️ 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

When the enforce_theme parameter in config.php is set to default, Nextcloud does not automatically adjust the theme (dark mode or light mode) based on the browser's settings.

Steps to reproduce

1. Set the enforce_theme parameter in config.php to default:
2. Open Nextcloud in Browser and confirm default theme is enforced at settings/user/theming
3. Observe that Nextcloud does not change its theme based on the browser's theme settings. (Dev Tools ⇾ Rendering ⇾ Emulate CSS media feature prefers-color-scheme)

Expected behavior

Nextcloud should automatically switch between dark mode and light mode based on the browser's theme settings when the enforce_theme parameter is set to default.

Installation method

None

Nextcloud Server version

29

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start with the enforce_theme setting in config.php and reproduce the behavior from settings/user/theming using Dev Tools' Rendering panel and the prefers-color-scheme emulation. Trace how the default theme is selected, then verify that switching the browser preference changes between dark and light themes as expected.