Feat : Added dark theme as per system settings [#1502]

[omjamnekar]

Add Automatic Dark Mode Based on System Preference [#1502]

This update introduces automatic dark mode that responds to users’ browser or operating system settings via the CSS media query @media (prefers-color-scheme: dark).

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Changes Implemented

• Added a dark color palette (80+ CSS variables) in variables.css
• Updated common.css and theme.css to style all UI elements for dark mode
• The interface now automatically switches between light and dark themes based on the system/browser setting
• Removed previous manual theme toggle button and related script (theme-toggle.js)
• Simplified logic to rely purely on native browser behavior — no user interaction or localStorage needed

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Benefits

• automatic dark mode (no configuration required)
• Power savings on AMOLED displays
• Consistent experience across desktop and mobile browsers

Night time

Desktop

Day time

[UlrichB22]

The change in app.py is obsolet and causes failing tests.