Add javascript dark toggle mode (#308)

Co-authored-by: Stefan van der Walt <stefanv@berkeley.edu>

assets/js/dark.js

@@ -0,0 +1,18 @@
+/*******************************************************************************
+ * Theme interaction
+ */
+
+var prefersDark = window.matchMedia("(prefers-color-scheme: dark)");
+
+/**
+ * set the the body theme to the one specified by the user browser
+ *
+ * @param {event} e
+ */
+function autoTheme(e) {
+  targetTheme = prefersDark.matches ? "dark" : "light";
+  document.documentElement.setAttribute("data-theme", targetTheme);
+}
+
+autoTheme();
+prefersDark.onchange = autoTheme;