Enhancement: Add dark mode toggle to web interface

[vultuk]

Description

Add a dark mode toggle to the web interface to improve user experience, especially for users working in low-light environments.

Current Behavior

The interface currently only supports a single theme without the ability to switch between light and dark modes.

Proposed Solution

• Add a toggle button in the header/settings area
• Implement CSS variables for theming
• Store user preference in localStorage
• Apply theme on page load based on saved preference
• Consider respecting system-wide dark mode preference as default

Benefits

• Better accessibility and user comfort
• Reduced eye strain in low-light conditions
• Modern UI/UX standard feature
• Improved user experience

Implementation Notes

• Use CSS custom properties for theme colors
• Add smooth transitions when switching themes
• Ensure all terminal colors remain visible in both themes