Update mkdocs.yml

mkdocs.yml

@@ -12,10 +12,10 @@ nav:
     - Get Started:
         - Your First Extension: your_first_extension.md
 theme:
-  name: 'readthedocs'
+  name: 'material'
   highlightjs: true
   language: 'en'
   custom_dir: custom_theme
 
 extra_css:
-  - css/extra.css
+  - css/extra.css