diff --git a/sphinx/application.py b/sphinx/application.py
index 840fc46b9..dec9c13c8 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -472,8 +472,11 @@ class Sphinx(object):
 
     def add_stylesheet(self, filename):
         from sphinx.builders.html import StandaloneHTMLBuilder
-        StandaloneHTMLBuilder.css_files.append(
-            posixpath.join('_static', filename))
+        if '://' in filename:
+            StandaloneHTMLBuilder.css_files.append(filename)
+        else:
+            StandaloneHTMLBuilder.css_files.append(
+                posixpath.join('_static', filename))
 
     def add_lexer(self, alias, lexer):
         from sphinx.highlighting import lexers