style: fix dark mode accessibility issues (#422)

[bhatiakavisha]

Description

This PR addresses Issue #422 by resolving multiple text readability, border contrast, and dark mode accessibility issues across the application.

Key Changes

• Replaced hardcoded colors (#555, #1f2937, #111, #ccc, #e1e5e9) with theme-aware variables.
• Improved hero section contrast on Syllabus, FAQ, and PYQs pages.
• Updated forms, dropdowns, cards, and feedback components to respect theme variables.
• Fixed readability of Mind Map info nodes and Contribute page code blocks.
• Improved consistency between light and dark themes.

Files Updated

• Syllabus.css
• About.css
• Privacy.css
• Contact.css
• Faq.css
• Feedback.css
• SubmitFeedback.css
• Notes.css
• PYQs.css
• MindMapEditor.css
• Contribute.css

Verification

• Verified affected pages in both light and dark modes.
• Confirmed improved contrast and readability.
• Kept changes limited to Issue Add Dark Mode Support for Better Visibility and User Experience #422 scope.

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