Add html_theme to read the docs

Builds are failing on read the docs because html_theme is not defined,
this was being done on purpose (old style) but no longer seems to work.

Example of failing build:
https://readthedocs.org/projects/pre-commit-vauxoo/builds/21727547/

docs/conf.py

@@ -27,12 +27,8 @@
     "issue": ("https://github.com/Vauxoo/pre-commit-vauxoo/issues/%s", "#"),
     "pr": ("https://github.com/Vauxoo/pre-commit-vauxoo/pull/%s", "PR #"),
 }
-# on_rtd is whether we are on readthedocs.org
-on_rtd = os.environ.get("READTHEDOCS", None) == "True"
-
-if not on_rtd:  # only set the theme if we're building docs locally
-    html_theme = "sphinx_rtd_theme"
 
+html_theme = "sphinx_rtd_theme"
 html_use_smartypants = True
 html_last_updated_fmt = "%b %d, %Y"
 html_split_index = False