Add javascript dark toggle mode

I initially tried to use the themeswitcher from:
https://github.com/pydata/pydata-sphinx-theme/blob/main/src/pydata_sphinx_theme/assets/scripts/pydata-sphinx-theme.js

assets/js/dark.js

@@ -0,0 +1,11 @@
+window
+  .matchMedia("(prefers-color-scheme: dark)")
+  .addEventListener("change", ({ matches }) => {
+    if (matches) {
+      targetTheme = "dark";
+    } else {
+      targetTheme = "light";
+    }
+    document.documentElement.setAttribute("data-theme", targetTheme);
+    localStorage.setItem("theme", targetTheme);
+  });