feat: add custom scrollbar with existing theme

[Anmol-Baranwal]

Related Issue(s)

Closes #108

Proposed Changes

• I have made changes in index.scss.
• CSS variables have been used to facilitate testing.
• Cross-compatibility issues for browsers such as Webkit-based browsers, Firefox, and Internet Explorer and Microsoft Edge have been handled.
• Extra code has been commented for others to experiment with when they utilize the code.

Type of Change

• [ ] New Feature (Landing Page)
• [ ] New Feature (App)
• [ ] Bug (Landing Page)
• [ ] Bug (App)
• [ ] New Component (App)
• [x] Enhancement
• [ ] Documentation / ReadMe / Workflows
• Other (If not in above options):

Your Idea for Level (GSSoC)

• [ ] Level 1
• [ ] Level 2
• [ ] Level 3

Could be level 2 or level 3

Screenshots (if applicable)

Additional Notes

[github-actions[bot]]

Thank you, @Anmol-Baranwal, for creating this pull request and contributing to GitHub-ReadMe! 💗

Our review team will thoroughly review the pull request and will reach out to you soon! 😇 Please make sure you have marked all the completed tasks as done. ✅ We appreciate your patience and contribution! 😀

[Anmol-Baranwal]

@praveenscience Could you please review this pull request :)