Add dark/light mode to the website

[harshita9621]

Issue Number

fixes #22

Describe the changes you've made

Add dark and light mode on the website.

Describe if there is any unusual behaviour of your code(Write NA if there isn't)

NA

Additional context (OPTIONAL)

Checklist

• I have added my name in the contributors list at the end of README.md file.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• The title of my pull request is a short description of the requested changes.
• Open Source Program names(OPTIONAL: If you participated in any open-source program then please mention the program name here)

[QAZIMAAZARSHAD]

Resolve conflicts,

Also, it doesn't look good beside the scroll bar.

Improve the design of the toggle make it small and add it below the navbar.