I'm tempted to add a selection of different backgrounds to the Change Theme page. The game supports both centred and tiled images, among a few other common configurations.

It may also be possible to allow players to upload an image from their computer, and use that. In the future, perhaps there could even be a public gallery of custom background images to use with the game -- for now, it would be strictly private.

As you can see, the background image we have at the moment has a fade-to-black effect applied to it, to stop it looking awful when you resize the browser window beyond the dimensions of the background image itself. This is a common trick. I may apply this effect automatically on all user-uploaded images, or I could leave it solely as an option on the upload form.

I'll probably do it like this:

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#1. Choose an image file to upload.

#2. Set whether image is tiled, centred, etc.

#3. If not tiled, set whether image should have the fade-to-black applied.

The image dimensions will be problematic, but I think the best way might be to set a max width and height, and if the image is set to fade-to-black, crop it to the standard size, as the rest would be black anyway.

Alternatively, and more simply, the image could have to be a specific size, and that would be enough to do tiled patterns, plus the default fading to black effect.

Feedback welcome, as always.