[fix] Improve dark-theme contrast on /about page

[MahalaxmiKannan]

Closes #173

Summary of Changes

• Added scoped dark-mode overrides to about.css to improve paragraph/list/heading contrast on the /about page.
• Changes are minimal and only affect dark theme visuals (scoped under .dark) so light theme and other pages are unchanged.
• Purpose: improve readability and accessibility for the sections "Our Mission", "Our Vision", and "Why KrishiSetu?".

Type of Change

• 🐛 Bug fix
• ✨ New feature / enhancement
• 🧹 Refactor / code cleanup
• 📄 Documentation update
• 🧪 Test addition or update
• ⚙️ Configuration / tooling change

Testing Steps

1. Install and run locally:

   npm install
   npm run dev

2. Open: http://localhost:5001/about
3. Enable dark mode (use the theme toggle in the UI) and verify:
   • Paragraph text under Our Mission and Our Vision is clearly readable.
   • List items under Why KrishiSetu? are legible and hover states remain functional.
   • Headings and the back link are visually consistent and accessible.
4. Quick check: compare before/after screenshots to confirm improved contrast.

Environment tested on:

• OS: Windows 11
• Browser: Chrome (latest)
• Node version: 20.19.0

Demo

Checklist

• My code follows the project's coding style and conventions
• I have tested my changes locally (npm install + npm run dev)
• I have linked the relevant issue above
• I have not introduced any new linting errors or warnings
• I have updated documentation if needed
• My branch is up to date with main

GSSoC Declaration

• This PR is submitted under GSSoC 2026
• I have read and followed the Contributing Guidelines
• I have not plagiarised any content

Files changed:

• about.css

[MahalaxmiKannan]

@aditiraj2006 Kindly review the pr !