Implement live preview for the dark mode setting in the user profile

Problem/Motivation

The dark mode setting per user brings in a new data-color-scheme for the html tag. When the user changes the setting by selecting a different radio button for that mode, we should get a live preview for that, similar to how Gin had done it in the old days.

Not sure if this belongs to the default_admin theme, so please feel free to move it elsewhere, just in case.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet