Feat: Add persistent dark theme toggle with localStorage state caching

[Siddh2024]

Summary

This Pull Request introduces a native, persistent Dark Mode toggle feature across the DevPath platform. To achieve this, core color tokens (background surfaces, card containers, tech stack badges, and typography) have been refactored into global CSS variables. A lightweight JavaScript event handler handles toggling a data-theme="dark" attribute on the document root, and the selected preference is securely cached in localStorage to ensure continuity across page reloads and transitions.

Related Issue

Closes #318

Visuals (Screenshots/Videos):

Local Dark Mode Previews:

Checklist [required]

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings

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[Siddh2024]

@komalharshita , kindly approve my pr request ma'am.