Fix #3754: HTML builder crashes if HTML theme appends own stylesheets

CHANGES

@@ -13,6 +13,8 @@ Features added
 Bugs fixed
 ----------
 
+* #3754: HTML builder crashes if HTML theme appends own stylesheets
+
 Testing
 --------
 

sphinx/builders/html.py

@@ -121,6 +121,7 @@ class CSSContainer(list):
                       RemovedInSphinx20Warning)
         for item in other:
             self.append(item)
+        return self
 
     def __add__(self, other):
         ret = CSSContainer(self)