Use sphinx_rtd_theme when building docs locally

Default theme that Sphinx uses is not consistent with what we have at
https://tahoe-lafs.readthedocs.io.  Being consistent with rtd might
help future doc writers.

docs/conf.py

@@ -28,7 +28,7 @@ import os
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['recommonmark']
+extensions = ['recommonmark', 'sphinx_rtd_theme']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -107,7 +107,7 @@ todo_include_todos = False
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'default'
+html_theme = 'sphinx_rtd_theme'
 
 # 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

tox.ini

@@ -230,6 +230,7 @@ deps =
      sphinx
      docutils==0.12
      recommonmark
+     sphinx_rtd_theme
 # normal install is not needed for docs, and slows things down
 skip_install = True
 commands =