Apply default_mode to the html data-theme attribute (#1663)

Author: char101

src/pydata_sphinx_theme/theme/pydata_sphinx_theme/layout.html

@@ -14,7 +14,7 @@
   {# https://github.com/pydata/pydata-sphinx-theme/pull/1045 #}
   <script data-cfasync="false">
     document.documentElement.dataset.mode = localStorage.getItem("mode") || "{{ default_mode }}";
-    document.documentElement.dataset.theme = localStorage.getItem("theme") || "light";
+    document.documentElement.dataset.theme = localStorage.getItem("theme") || "{{ default_mode }}";
   </script>
   {{ _webpack.head_pre_assets() }}
   {{ _webpack.head_pre_icons() }}