added dark/light mode

[harshita0802goswami]

Issue Number

fixes #

ISSUE #98

Describe the changes you've made

A clear and concise description of what you have done to successfully close your assigned issue. Any new files? or anything you feel to let us know!
Dark and light mode added

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

A clear and concise description of it.
NA

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.
• [x ] My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [ x] I have added tests to cover my changes.
• [ x] All new and existing tests passed.
• [ x] 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]

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