DOC: Fix custom css code - the previous approach seems to have caused

something to break in a recent sphinx / sphinx-rtd-theme version [skip-ci]

doc/conf.py

@@ -152,11 +152,9 @@ html_theme = 'sphinx_rtd_theme'
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
-html_context = {
-    'css_files': [
-        '_static/theme_overrides.css',  # overrides for wide tables in RTD theme
-        ],
-    }
+html_css_files = [
+    'theme_overrides.css',  # overrides for wide tables in RTD theme
+]
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied