use DEFAULT_STATIC_FILES_PATH to find css files

get_ipython_package_dir() does not necessarily contain the files, e.g.
in distribution packages which share files with python3 in a different
folder.

IPython/nbconvert/preprocessors/csshtmlheader.py

@@ -87,8 +87,9 b' class CSSHTMLHeaderPreprocessor(Preprocessor):'
         header = []
         
         #Construct path to IPy CSS
-        sheet_filename = os.path.join(path.get_ipython_package_dir(), 
-            'html', 'static', 'style', 'style.min.css')
+        from IPython.html import DEFAULT_STATIC_FILES_PATH
+        sheet_filename = os.path.join(DEFAULT_STATIC_FILES_PATH,
+            'style', 'style.min.css')
         
         #Load style CSS file.
         with io.open(sheet_filename, encoding='utf-8') as file: