Implement Dark Theme based on device settings

Dark theme implementation is critical for a website often used on mobile devices by drivers in a dark vehicle or environment.

This has been something I've received lots of compliments about having it available on my 3rd party UI.

No files or tests are identified in the issue. Start by locating the React frontend's theme and device-preference entry points; done means the site follows device dark/light settings consistently for the mobile use case.

Written by the indexing model from the issue text.