From 62614afb8364e6722e6a6e95a4a26be5c07a9cfb Mon Sep 17 00:00:00 2001
From: Takeshi KOMIYA <i.tkomiya@gmail.com>
Date: Wed, 17 May 2017 23:24:13 +0900
Subject: [PATCH] Fix #3754: HTML builder crashes if HTML theme appends own
 stylesheets

---
 CHANGES                 | 2 ++
 sphinx/builders/html.py | 1 +
 2 files changed, 3 insertions(+)

diff --git a/CHANGES b/CHANGES
index 689b42f9c..6b09fec3d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -13,6 +13,8 @@ Features added
 Bugs fixed
 ----------
 
+* #3754: HTML builder crashes if HTML theme appends own stylesheets
+
 Testing
 --------
 
diff --git a/sphinx/builders/html.py b/sphinx/builders/html.py
index c5c186586..67618764a 100644
--- a/sphinx/builders/html.py
+++ b/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)