Add Dark Mode Support to CSSUI

[Shreyhac]

Currently, the CSSUI library provides a clean, modern design with light mode as the default theme. However, many users prefer dark mode for better accessibility, visual comfort, and energy savings on OLED displays. To enhance the user experience and extend the flexibility of the library, I propose adding an optional dark mode.

Toggle Option: Include a button or system preference detection to toggle between light and dark mode.
CSS Variables: Utilize CSS variables to define colors, ensuring easy customization and maintenance.
Smooth Transition: Ensure a smooth color transition when switching between modes for a better user experience.
Browser Compatibility: The dark mode should work seamlessly across all browsers currently supported by CSSUI (Edge 79+, Firefox 49+, Chrome 12+, and Safari 10+).
Accessibility Considerations: Ensure contrast ratios meet accessibility standards, especially for text and essential elements in dark mode.
Documentation Update: Add instructions to the documentation, explaining how to enable and customize dark mode for developers using the library.