Feature: #104868 - Add color scheme switching

See forge#104868

Description

Options have been added to switch between the available color schemes in TYPO3. A set of buttons for each available color scheme in the user dropdown at the top right and a setting in User Settings.

As the dark color scheme is currently regarded experimental until further notice, color scheme switching logic is currently hidden behind the UserTS setting setup.fields.colorScheme.disabled.

Impact

It is now possible to switch to an automatic, light or dark color scheme for use in the backend.