style(main): update syntax highlighting style to default

Changed the syntax highlighting style from 'github' to 'default' in the `read_root` function to ensure consistent styling across different environments. This change affects the CSS used for code blocks in the generated HTML content, providing a more neutral appearance.

No functional impact on the application logic or performance.

main.py

@@ -147,7 +147,7 @@ async def read_root():
         custom_css = css_file.read_text(encoding='utf-8') if css_file.exists() else ""
         
         # CSS pour la coloration syntaxique
-        code_css = HtmlFormatter(style='github').get_style_defs('.codehilite')
+        code_css = HtmlFormatter(style='default').get_style_defs('.codehilite')
         
         # Construction de la page HTML
         html_content = f"""