Add dark mode support and chart appearance customization

Provide a user-configurable dark mode and basic chart appearance customization to reduce eye strain during extended trading sessions. Users can toggle between light and dark themes via a persistent setting in the UI (and option to follow system theme). Dark theme should apply consistently across the application chrome, menus, dialogs, and chart background/axes/labels. Additionally, allow configurable chart appearance options such as background color (light/dark/auto), grid visibility, and color palettes for price series and indicators. The feature should persist user preferences across sessions and not affect data or trading functionality.