Add theme toggle and Monospace font

[igorcosta]

Implements a theme toggle feature, updates the page's font to Monospace by GitHub, and introduces a futuristic simple background.

• Theme Toggle: Adds a button to toggle between dark and light themes using JavaScript. The theme-toggle.js script handles the theme switching and stores the user's preference in local storage. The styles.css file defines the color schemes for both themes.
• Font and Styling: Integrates the 'Source Code Pro' font from Google Fonts, applied globally to the page via CSS in styles.css, ensuring all text uses this Monospace font by GitHub.
• Background and Button Styling: Enhances the page's aesthetic with a simple and futuristic background. Styles the theme toggle button for better visibility and interaction, including hover effects, fixed positioning, and transition animations in styles.css.

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