Refactor: Move open-source.html styles into a separate CSS file

[Nikhilrsingh]

Description:
• Currently, styles for open-source.html are inside the main/global CSS file.
• This makes the file large and harder to maintain.
• Proposal: Create a dedicated CSS file (open-source.css) and link it in open-source.html.
• Benefit: Cleaner structure, easier future modifications.

Benefits:
• Cleaner code organization
• Easier maintainability
• Better scalability if page-specific styles grow

[github-actions]

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[Nikhilrsingh]

@supriya46788 Hey, I would like to work on this issue as part of GSSoC'25. Kindly assign it to me and please add the GSSoC’25 labels (and any other labels you find appropriate). Thank you!