Introduces a responsive theme toggle button with styles for desktop and mobile views in `theme-toggle.css`. Updates `BitcoinProgressBar.js` to support dynamic theme changes and adds a new `updateTheme` method. Enhances `main.js` for theme management based on user preferences in `localStorage`. Modifies `base.html` and other HTML files to include the theme toggle button and necessary scripts. Introduces `theme.js` for managing theme constants and applying the DeepSea theme.
