[Feature] Implement dark mode

As a user, I want to switch between light and dark themes, so that I can reduce eye strain and work comfortably in different lighting conditions.

As a user, I want a dark mode option, so that I can use the app comfortably at night without bright screen glare.

As a user, I want to choose my preferred visual theme, so that I can customize the interface to match my personal preference and system settings.