Customizing the party is a new feature and there might still be a few issues with it. The options should all default to random upon first installing/updating the wallpaper.

I tried to get the characters to update in real time but there could be some issues depending on the order they're changed, it was a pretty quick and dirty update I did in an afternoon.

It should update all character slots correctly if you set the characters and then change to a different wallpaper and back though.