Added documentation for Dark theme functionality

[Shivam-AfA]

Signed-off-by: Shivam-AfA 88040318+Shivam-AfA@users.noreply.github.com

Notes for Reviewers

This PR fixes #51 I have updated the contributing.md file with documentation on the working of light and dark theme.

Signed commits

• [ ] Yes, I signed my commits.

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[Shivam-AfA]

@leecalcote Reviews needed.