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)
• 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]

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[Anmol-Baranwal]

@praveenscience
Could you please review this pull request :)