Fix #3683: Failed to build document if builder.css_file.insert() is called

CHANGES

@@ -26,6 +26,7 @@ Bugs fixed
   ``latexmk``) aborted earlier in case of LaTeX errors than was the case with
   1.5 series, due to hard-coded usage of ``--halt-on-error`` option. (refs #3695)
 * #3683: sphinx.websupport module is not provided by default
+* #3683: Failed to build document if builder.css_file.insert() is called
 
 Testing
 --------

sphinx/builders/html.py

@@ -100,6 +100,12 @@ class CSSContainer(list):
         else:
             super(CSSContainer, self).append(Stylesheet(obj, None, 'stylesheet'))
 
+    def insert(self, index, other):
+        warnings.warn('builder.css_files is deprecated. '
+                      'Please use app.add_stylesheet() instead.',
+                      RemovedInSphinx20Warning)
+        self.insert(index, other)
+
     def extend(self, other):
         warnings.warn('builder.css_files is deprecated. '
                       'Please use app.add_stylesheet() instead.',