Allow specification of full highlight class in CSSHTMLHeader

IPython/nbconvert/transformers/csshtmlheader.py

@@ -35,7 +35,7 b' class CSSHtmlHeaderTransformer(ActivatableTransformer):'
 
     header = []
 
-    highlight_class = Unicode('highlight', config=True,
+    highlight_class = Unicode('.highlight', config=True,
                               help="CSS highlight class identifier")
 
     def __init__(self, config=None, **kw):
@@ -103,7 +103,7 b' class CSSHtmlHeaderTransformer(ActivatableTransformer):'
 
         #Add pygments CSS
         formatter = HtmlFormatter()
-        pygments_css = formatter.get_style_defs('.' + self.highlight_class)
+        pygments_css = formatter.get_style_defs(self.highlight_class)
         header.append(pygments_css)
 
         #Set header