refactor(doc): Use default theme for local builds

The sphinx_rtd_theme takes up about 7.8MB because it includes a lot of
fonts. The default theme only takes 600KB

doc/conf.py.in

@@ -12,7 +12,7 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
-# import os
+import os
 # import sys
 # sys.path.insert(0, os.path.abspath('.'))
 
@@ -71,10 +71,16 @@ pygments_style = None
 
 # -- Options for HTML output -------------------------------------------------
 
+# is whether we are on readthedocs.io
+on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
+
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
+if on_rtd:
   html_theme = 'sphinx_rtd_theme'
+else:
+  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