replace deprecated extra_css_file option with html_css_files

By that, the html_static_path is automatically prepended.

This fixes:
 WARNING: The extra_css_file option is deprecated, use the
 html_css_files option from Sphinx instead.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/conf.py

@@ -252,11 +252,9 @@
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'kasdoc'
 
-html_context = {
-    'extra_css_files': [
-        '_static/theme_overrides.css',  # override wide tables in RTD theme
-    ],
-}
+html_css_files = [
+    'theme_overrides.css',  # override wide tables in RTD theme
+]
 
 # -- Options for LaTeX output ---------------------------------------------