Ability for admins to enable/disable Dark mode configuration

**Is your feature request related to a problem? Please describe.**
Your whitelabled Metabase might not play nicely with the colors of Dark mode, and currently there's no admin setting or env var to disable users (that can be end-consumers/customers) from enabling Dark mode.

**Describe the solution you'd like**
Some way to control this

**Describe alternatives you've considered**
N/A

**How important is this feature to you?**
Requested by a customer, internal ticket [here]()

**Additional context**
N/A

The issue does not identify any files, tests, or existing dark-mode configuration entry points. Start by tracing how dark mode is currently enabled and where admin settings are defined; done should include an agreed control that prevents end-consumers from enabling dark mode and coverage for the resulting behavior.

Written by the indexing model from the issue text.