Add dark mode to the admin interface

As an admin user, I want to toggle a dark mode in the admin interface so I can reduce eye strain and work comfortably in low-light environments. Description: Add a dark mode theme to the admin interface, including a user-accessible toggle and settings to remember preferences. Support system theme detection with an override. Ensure consistent styling across all components without affecting functionality. Problem: Extended use of the current light theme causes eye strain, especially at night or in low-light settings. Users lack a way to adjust the interface to their preferences, impacting comfort and productivity. Benefit: Improves accessibility and comfort, reduces fatigue, and aligns with modern UI expectations. Enhances usability for admins working long hours, and can lower perceived glare while maintaining legibility.