add dark mode js demo on chapter 3 slide

slides/223015c/03-interaktivitaet-javascript.md

@@ -755,6 +755,8 @@ Formular-Validierung ohne Reload
 
 # Beispiel: Toggle Dark Mode
 
+![bg right:35%](./assets/demos/js-darkmode.png)
+
 ```javascript
 const toggleButton = document.querySelector("#darkModeToggle");
 const body = document.body;