Remove CSS override that doesn't exist.

Also remove static path file,
and run our tests with warnings enabled since we can now.

docs/conf.py

@@ -71,7 +71,7 @@ locale_dirs = [
 gettext_compact = False
 
 html_theme = 'sphinx_rtd_theme'
-html_static_path = ['_static']
+#html_static_path = ['_static']
 html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 html_logo = 'img/logo.svg'
 html_theme_options = {
@@ -79,6 +79,3 @@ html_theme_options = {
     'display_version': False,
 }
 
-
-def setup(app):
-    app.add_stylesheet('custom.css')

tox.ini

@@ -25,7 +25,7 @@ commands =
 description = build readthedocs documentation
 changedir = {toxinidir}/docs
 commands =
-    sphinx-build -b html -d {envtmpdir}/doctrees .  {envtmpdir}/html
+    sphinx-build -W -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
 
 [testenv:docs-lint]
 description = run linter (rstcheck) to ensure there aren't errors on our docs