Add dark mode toggle to the first-contributions website

🐞 Problem
The website currently only supports a light theme, which might not be comfortable for users working in low-light environments.

🎯 Goal
Improve user experience and accessibility by adding a dark mode toggle, which is especially useful for contributors spending long hours reading or contributing at night.

💡 Possible solutions
Implement a CSS-based dark mode toggle with local storage support to remember the user's preference.

📋 Steps to solve the problem

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