replace html_context with html_css_files in configuration

docs/source/conf.py

@@ -100,12 +100,7 @@ html_theme = 'sphinx_rtd_theme'
 # relative to this directory. They are copied after the builtin static files,
 # 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']
 
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.