Hii thanks for reaching out! I hadn't expected anyone to try adding their own backgrounds, so I am unfortunately unable to recover your coins as that feature is unsupported. However I have added a new feature that might be able to fix your problem!
With the new restart functionality, you can reset your coins and items back to zero, which should fix your coins. If you use the restart function in the app every time you add a new background to it, it should work smoothly!