docs: Set html_theme on rtd builds

Sets `html_theme` when building on rtd since the default theme was
recently removed from the build infrastructure and builds now fail
without `html_theme` set.

Ref: readthedocs/readthedocs.org#10638

[skip ci]

docs/conf.py

@@ -70,11 +70,12 @@
      'Miscellaneous'),
 ]
 
+html_theme = "sphinx_rtd_theme"
+
 # on_rtd is whether we are on readthedocs.org
 on_rtd = getenv("READTHEDOCS", None) == "True"
 if not on_rtd:
     import sphinx_rtd_theme
-    html_theme = "sphinx_rtd_theme"
     html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 
 exclude_patterns = [u'_build', 'Thumbs.db', '.DS_Store']