I don't have an exact link for this as I played around in the settings but you should be able to use the UI Series portion of the VN manual for the basic idea of how to install the images and using image maps but this is how I do it.
Go to Database then choose Common Events on the upper left. That'll open up a bunch of things under Records in the box below it. You'll want to find Message Box as that's where you'll be able to adjust the various settings (mine was under Beginner's Guide). The main one to get everything to "fit" properly is the image map options. Look for Hotspots which should have a number. Clicking that will bring you into a new window which will show the image you're using and existing hotspots. Simply adjust them as needed (right clicking on a hotspot will allow you to go into properties which you can use to adjust where the hotspot sends you).
So after installing the image it was:
Database -> Common Events ->Beginner's Guide -> Message Box -> Hotspots
The Dark Toon art style is now available as Steam branches. (Both for the main game and the demo)
There are 2 versions. One only affects combat, cutscene, and portraits. The other one changes more.
So, technically, the game now has 7 versions on steam.