Fix rtd search function

By adding the theme to the extension list in conf.py.

docs/conf.py

@@ -28,7 +28,8 @@ import sys, os
 extensions = [
   'sphinx.ext.todo', 
   'sphinx_copybutton',
-  'sphinx_tabs.tabs'
+  'sphinx_tabs.tabs',
+  'sphinx_rtd_theme'
 ]
 
 # Add any paths that contain templates here, relative to this directory.