Revert "switch to the rtd theme"

This reverts commit a1466466ae.

source/conf.py

@@ -15,7 +15,6 @@
 
 import sys
 import os
-import sphinx_rtd_theme
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -103,7 +102,7 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'sphinx_rtd_theme'
+html_theme = 'default'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
@@ -111,7 +110,7 @@ html_theme = 'sphinx_rtd_theme'
 #html_theme_options = {}
 
 # Add any paths that contain custom themes here, relative to this directory.
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+#html_theme_path = []
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".