Added dark mode and persisting it with the help of useLocalStorage hook

[UsamaLakhwera]

🛠️ Fixes Issue #12 :

"This PR Adds "Dark Mode" to the app"

• Made Css changes in App.css
• Made css changes in index.css
• Added a dark mode toggle button in nav bar which conditionally changes the dark mode
• Used local storage to persist the dark mode

✔️ Check List (Check all the applicable boxes)

• [✔️] My code follows the code style of this project.
• [✔️] This PR does not contain plagiarized content.
• [✔️] The title of my pull request is a short description of the requested changes.
• I am a participant of Hacktoberfest 2022.

📷 Screenshots

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