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Someone help, how i set a custom background in dig for each system?
In Settings, go into Themes -> Edit Themes for the theme you want to edit. In there, in the upper left corner, you'll see the theme name and a menu that says "Defaults". Delect the "Defaults" menu. There will be a number of options - Systems is the screen that lets you choose what system you want to play, Games is, well, the menu that lets you choose your games for your system, and Game Info gives you...info about the game.
Inside of each there is an option for "System backgrounds folder" - change that to "Folder" and select the folder where your backgrounds are stored.
Now, each system needs a specific file name - for the exact file name structure, snag an existing theme and it should be labeled properly.