Reinstate html_css_files parameter for handling top banner of the docs

Partial revert of 5e38721

Author: DelazJ

conf.py

@@ -163,6 +163,10 @@
     'isLtr': False,
 }
 
+# Add custom CSS when a top bar is needed to be shown (for testing or outdated versions)
+if html_context['isTesting'] or html_context['outdated']:
+    html_css_files = ['css/qgis_topbar.css']
+
 # Add custom tags to allow display of text based on the branch status
 if html_context['isTesting']:
     tags.add('testing')