Added dark mode

[divya-nshi]

Description

Made changes in App.jsx and components to add the dark mode

Related Issue

Motivation and Context

How Has This Been Tested?

Checklist:

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

Screenshots (if appropriate):

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