Update deprecated api call

conf.py

@@ -148,7 +148,7 @@ html_favicon = '_static/images/ecam_logo.ico'
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['./_static']
 def setup(app):
-    app.add_stylesheet("theme_extra.css")
+    app.add_css_file("theme_extra.css")
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.