docs: Make background at least one screen tall

Otherwise this would look ugly by stopping the gradient after the
content, so in e.g. the `end` or `false` page it would leave an ugly stripe at
the bottom.

doc_src/python_docs_theme/static/pydoctheme.css

@@ -2,6 +2,7 @@
 
 html {
     background: none;
+    min-height: 100%;
 }
 
 body {