doc: ensure the footer has the same color than the background.

mkdocs.yml

@@ -44,8 +44,6 @@ theme:
     - content.action.view
     - content.code.annotate
     - content.code.copy
-    - navigation.expand
-    - navigation.footer
     - navigation.indexes
     - navigation.sections
     - navigation.tabs
@@ -54,6 +52,9 @@ theme:
     - search.share
     - search.suggest
 
+extra_css:
+  - assets/stylesheets/extra.css
+
 # Plugins
 plugins:
   - search