Ah it should be `scroll="viewport"` as a string. The fact that it doesn't complain about Viewport is just 'cause you're setting it to the name of a class, which does exist, but isn't what the screen is expecting.
Have you updated ICON_FOLDER = "backend/controller_support/controller_ui/" at the top of controller_config.rpy to point to where you have the controller icons? It looks like you've put the files into your own subfolder, so you'll need to update the path where it can find the icons as well.
