[ui-upgrade]: Adding a option of Dark-theme to users. [hacktober fest]

Describe the bug Adding a button which when clicked will change the theme of website to dark theme.

To Reproduce Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior A clear and concise description of what you expected to happen.

Screenshots If applicable, add screenshots to help explain your problem.

Additional context Add any other context about the problem here.

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