Consider allowing site admins to set global values for light/dark mode

Problem/Motivation

In #3606655: Make dark mode a first class citizen of Drupal core, we're creating a per user setting that allows users to set their dark/light mode preference.

However, there might be use cases where admins want to

• Restrict the ability for users to change their preference (and set a global default)
• Set the global default (but still allow users to change their preference)

Remaining tasks

1. Identify these use cases and make sure we want to support them
2. Make sure there's no accessibility issues in allowing admins to restrict it (I don't think there are)
3. Figure out what the UI will look like and where it will live
4. Do the work.