More background photos here. https://www.dropbox.com/sh/okuugwnbu5ip1so/AAB1CJwAjGgwGbVEJdSuKsfda?dl=0
Go to View > Wallpaper to bring up the gui manager and change wallpaper settings.
All custom themes added to the themes folder will be auto deleted on upgrades by Anki. Make sure to backup custom themes. This is not the same as background images in the user_files folder which is save from upgrades.