diff --git a/sphinx/application.py b/sphinx/application.py
index 80c96d280..6f1418b2f 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -738,15 +738,15 @@ class Sphinx(object):
     def add_stylesheet(self, filename, alternate=False, title=None):
         # type: (unicode, bool, unicode) -> None
         logger.debug('[app] adding stylesheet: %r', filename)
-        from sphinx.builders.html import StandaloneHTMLBuilder
-        props = {'rel': 'stylesheet',
-                 'filename': filename,
-                 'title': title}  # type: Dict[unicode, unicode]
+        from sphinx.builders.html import StandaloneHTMLBuilder, Stylesheet
         if '://' not in filename:
-            props['filename'] = posixpath.join('_static', filename)
+            filename = posixpath.join('_static', filename)
         if alternate:
-            props['rel'] = 'alternate stylesheet'
-        StandaloneHTMLBuilder.css_files.append(props)
+            rel = u'alternate stylesheet'
+        else:
+            rel = u'stylesheet'
+        css = Stylesheet(filename, title, rel)  # type: ignore
+        StandaloneHTMLBuilder.css_files.append(css)
 
     def add_latex_package(self, packagename, options=None):
         # type: (unicode, unicode) -> None
diff --git a/sphinx/builders/html.py b/sphinx/builders/html.py
index 7bfce0ad6..7c9bc8b62 100644
--- a/sphinx/builders/html.py
+++ b/sphinx/builders/html.py
@@ -86,6 +86,23 @@ def get_stable_hash(obj):
     return md5(text_type(obj).encode('utf8')).hexdigest()
 
 
+class Stylesheet(text_type):
+    """The metadata of stylesheet.
+
+    To keep compatibility with old themes, an instance of stylesheet behaves as
+    its filename (str).
+    """
+
+    def __new__(cls, filename, title, rel):
+        # type: (unicode, unicode, unicode) -> None
+        self = text_type.__new__(cls, filename)  # type: ignore
+        self.filename = filename
+        self.title = title
+        self.rel = rel
+
+        return self
+
+
 class StandaloneHTMLBuilder(Builder):
     """
     Builds standalone HTML docs.