Fix documentation CSS. (#956)

This change fixes the documentation generation so that the CSS
styles start working again.

Documentation/source/conf.py

@@ -136,12 +136,7 @@
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
-# https://rackerlabs.github.io/docs-rackspace/tools/rtd-tables.html
-html_context = {
-    'css_files': [
-        '_static/theme_overrides.css',  # override wide tables in RTD theme
-        ],
-     }
+html_css_files = ['theme_overrides.css']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.