Use alabaster theme everwhere. (#8021)

'sphinx_rtd_theme' is no longer installed by default.

Author: tseaver

docs/conf.py

@@ -120,7 +120,7 @@
 
 # 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 = "alabaster"
 
 # 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