refactor: Standardize theme structure and update configs

- Restructured 'minimal' theme to use 'static/css' and 'static/js' subdirectories for assets, aligning with qsgen3's expected theme structure.
- Moved 'minimaltemplate-v1.css' to 'themes/minimal/static/css/'.
- Moved JS files from 'themes/minimal/css/' to 'themes/minimal/static/js/'.
- Updated 'site.conf.example' to reflect 'minimal' theme usage and correct 'site_theme_css_file' path relative to the theme's static directory.
- Cleaned up redundant CSS variable definitions in 'bin/qsgen3' (_generate_index_page function).

site.conf.example

@@ -5,7 +5,8 @@
 site_name="My Example Site"
 site_tagline="An example site generated by qsgen3"
 site_url="http://localhost:8000" # Or your actual production URL
-site_theme="default" # Specifies the theme to use (e.g., default, dark)
+site_theme="minimal" # Specifies the theme to use (e.g., minimal, default)
+site_theme_css_file="css/minimaltemplate-v1.css" # Path to main CSS file, relative to THEME'S static/ directory
 
 # --- Paths (relative to project root) ---
 paths_content_dir="content"