[ENHANCEMENT]: Add Dark Mode Toggle for Better User Experience

Describe the enhancement
The current EDGEx project lacks a dark mode toggle, which is a widely requested feature to improve accessibility and user experience, especially for low-light environments.

Why is this important?

• Dark mode reduces eye strain for users browsing at night or in dark environments.
• It modernizes the UI and makes the app visually appealing to a broader audience.
• Many modern web apps support dark mode as a standard feature.

Proposed Solution

• Implement a dark mode toggle button (e.g., in the navbar or sidebar).
• Use CSS variables or Tailwind CSS classes to switch between light and dark themes dynamically.
• Store user preference in localStorage to persist the choice across sessions.
• Optionally, use the user’s OS color scheme preference (prefers-color-scheme media query) as the default.
• Test responsiveness and readability for both modes on all devices.

Steps to Implement:

1. Define CSS variables or Tailwind config for light and dark themes.
2. Add a toggle button in the UI that switches themes on click.
3. Update the app's root or body class to reflect the current theme.
4. Save user preference and load it on page load.
5. Ensure all components have styles for both themes.