Added dark mode button and dark mode theme to all necessary elements

[Polyzosk]

Description:
This pull request introduces full support for a Dark Mode theme, along with several UI improvements for better user experience and consistency. Below is a summary of the changes:

Key Features:

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Dark Mode Toggle:
Added a Dark Mode toggle button in the header for users to switch between Light and Dark modes seamlessly.

Dark Mode Styling:
Updated the styling of all major components to support Dark Mode:
Headers and Titles: Now adapt to Dark Mode with white text for better visibility.
Countdown Timer: Adjusted text color to ensure clear visibility in both Light and Dark modes.
Drop-down Menus: Fixed colors for both options and hovered items to match the active mode.
QNA Tables: Styled tables with appropriate background, text, and hover effects for Dark Mode.

Improved Consistency:
Ensured consistent colors for all UI elements, including buttons, borders, and hover states.

Performance:
Implemented smooth transitions for color changes between modes, ensuring a polished user experience.

Technical Details:
Modified the following components to enable Dark Mode:
App/index.js
Countdown/index.js
Header/index.js
Main/index.js
QNA.js
index.css

Utilized CSS transitions for smooth color changes and ensured compatibility with the Semantic UI framework.
Testing:
Manually tested in both Light and Dark modes to ensure:
Proper color contrast and readability.
Functional toggle behavior without breaking other features.
Consistent appearance across all major components.

[SafdarJamal]

Hi @Polyzosk

Thank you for your efforts in adding Dark Mode support! The implementation looks great, and the transitions between Light and Dark modes work seamlessly. I appreciate the work you've put into updating the styling for major components.

During testing, I noticed a few issues that need adjustments before we merge the PR:

• Quiz Answer Selection: When a user selects an answer in the quiz, there is no visual indication in Dark Mode, unlike in Light Mode. This makes it unclear which option has been chosen. Could you update the styling to ensure selected answers are visibly highlighted in Dark Mode as well?

• Selection Fields Visibility: On the main page, the selection fields (such as dropdowns) have a black background, which blends into the Dark Mode theme, making it difficult to differentiate them. Adjusting the background or adding a border/highlight effect would help improve visibility.

• Result Page Tabs: On the results page, the tabs (Stats, QNA) do not visibly show which one is active, unlike in Light Mode. Please adjust the styling so that the active tab stands out clearly in Dark Mode.

• Unexpected White Light on Quiz Page: There's a strange white light appearing on the right side of the main quiz page, which
  looks out of place in Dark Mode. Could you check and fix this issue?

Once these are addressed, we should be good to go! Let me know if you need any clarification.
Thanks again for your contribution.