After a long while not posting any new demos, I found some time to create a rough "background switcher". This one uses JS only to dynamically switch background colors and images, without tampering with Storyline output whatsoever. Samples for most use cases, and typical behaviours are included in the demo.
I'm attaching a Tempshare link for the time being. If it sparks your interest I could create a blog post about it.
Demo : BackgroundMe!