Update document theme and section headers

This commit updates the document theme. It is clear, simple, up-to-date
and beautiful. And this commit also updates section headers to fix the
document index.

Change-Id: Ia1e8e16e5f5ddb06e4c4ccb4979b0750b88a4db4
diff --git a/doc/source/conf.py b/doc/source/conf.py
index cbfcc09..f11d96a 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -27,6 +27,8 @@
 import subprocess
 import warnings
 
+import openstackdocstheme
+
 # Build the plugin registry
 def build_plugin_registry(app):
     root_dir = os.path.dirname(
@@ -117,7 +119,7 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'nature'
+html_theme = 'openstackdocs'
 
 # 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
@@ -125,7 +127,7 @@
 #html_theme_options = {}
 
 # Add any paths that contain custom themes here, relative to this directory.
-#html_theme_path = []
+html_theme_path = [openstackdocstheme.get_html_theme_path()]
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".