Fix dark-theme in html[data-theme=dark]-tags (#9200)

* Fix dark-theme in html tag

* Add to release notes

Author: prisae

doc/whats-new.rst

@@ -45,6 +45,8 @@ Bug fixes
   By `Justus Magin <https://github.com/keewis>`_.
 - Fiy static typing of tolerance arguments by allowing `str` type (:issue:`8892`, :pull:`9194`).
   By `Michael Niklas <https://github.com/headtr1ck>`_.
+- Dark themes are now properly detected for ``html[data-theme=dark]``-tags (:pull:`9200`).
+  By `Dieter Werthmüller <https://github.com/prisae>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/static/css/style.css

@@ -14,6 +14,7 @@
 }
 
 html[theme=dark],
+html[data-theme=dark],
 body[data-theme=dark],
 body.vscode-dark {
   --xr-font-color0: rgba(255, 255, 255, 1);