Users who are not allowed to change the theme, cannot change the theme mode either

[nilmerg]

Describe the bug

A user who has only the theme mode selection available, can choose whatever they like but storing it has no effect.

To Reproduce

1. Go the general application configuration
2. Prohibit changing the theme, globally
3. Choose the Icinga default theme as default
4. Save
5. Go to the preferences
6. Switch to another theme mode
7. Save
8. Notice that the mode change has no effect

Expected behavior

The user should be able to change the theme mode.

Your Environment

• Icinga Web 2 version and modules (System - About): latest

Additional context

This is related to #5265, somewhat. Here, also the businessprocess module is the culprit. Enabling it, solves this issue. Having it disabled, authentication has not been performed at the stage the theme mode is evaluated, while theme selection has been disabled. (Which, if enabled, would also trigger authentication in time.)