The only problem I'm having with this theme is I have no idea how to get the background image to change to the right ones for each system. Mine is just stuck on the default.
It is very easy to change the background images, but you cannot do it within attract mode. You have to do it by renaming files in the downloaded folder.
The layout folder "Famicom Mini Ui" consists of two folders; systems and ui. In order to change the background image for each system, you can start by simply duplicating the "Famicom Mini Ui" folder and rename it as the prefered system you want to crate a layout for.
Say you wanted to create a layout for playstation, rename it as "Playstation Mini Ui" (you can rename it however you like).
After that enter the folder named "systems". Delete (or rename) the files called "default-menu.png" and "default.png". After that, the final step is to rename the the images you want to use in your layout. In this case rename "playstation-menu.png" to "default-menu.png" and "playstation.png" to "default.png".
After that you are set. Simply add the layout to your system and choose "Playstation Mini Ui" as the layout for your emulator within attract mode.
Edit: Sorry, this was terrible advice! Even though it kind of worked..The much simpler and more appropriate approach to get what you want, is to name the emulators/Displays within attract mode according to the case names used in file "layout.nut" (I believe they are capital sensitive).
Use the following naming conventions:
Playstation
Displays Menu
snes
famicom
Game Boy
Sega Genesis
If the name of the emulator doesn't match any of the names, the default background and settings will be used.