Introduce support for themes by adding a `theme` parameter to the site generation process and as an argument to the main script. Created separate CSS files for different themes and adjusted the structure to accommodate the style changes. Also, updated the use of assets with new SVG images and logos.
Refactored HTML templates to dynamically select the theme and updated path references to images accordingly. Improved the development experience by adding a default "plain" theme and enhancing command-line argument parsing for theme selection.
Closes#7
