I can see the files in the dir, but I can't possibly know why it's not working without seeing the how everything loads into the actual gallery. You would need to open browser developer tools > network inspector to see if things are being loaded properly and applying properly.
Browser developer tools > networks, let's see if your custom.css and image.jpg is loading.

If the css and image is loading, we need to check why they aren't applying, from the "Elements" tab.
