Dark Mode added

[divya-nshi]

#93 issue solved.

Description

I have successfully added the dark mode to the website.

Motivation and Context

Most of the users prefer dark mode.

Checklist:

• [ x] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] My changes generate no new warnings
• [ x] I have added tests that prove my fix is effective or that my feature works

Screenshots (if appropriate):

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