By analogy with the ImageCompare script, it allows you to look at wikis with a light and dark theme. To activate/click on some wiki items that overlay the slider - move the slider to another location. It opens with a key in the My Tools panel.
Could you not just use one of the pre-existing scripts which allows you to temporarily change the theme for a page? They don't provide a side-by-side comparison but I think toggling between themes while not changing/moving the page would accomplish a similar effect.
