Build Dark Mode with CSS Variables

**Title:** Implement Theme Switching with CSS Custom Properties

## Problem Statement

The application only supports a light theme, causing eye strain for users in low-light environments. Modern web applications should offer dark mode as a standard feature.

## Proposed Solution

Implement a complete theming system using CSS variables with:

* Light and dark color schemes
* Toggle button with smooth transition
* System preference detection
* Persistent theme choice
* All elements properly themed
* Smooth color transitions