I never really thought of it...but I'd also like to know. Looking at the guide edit page, I don't see anywhere that allows you to change the background. =[
When you are logged in and viewing your guide, on the top left underneath your profile picture at the top of the guide, there should be three buttons, one of which says "Edit". Click that.
Once you're editing, the same rules apply as when you originally created it. "Save" will save your progress as a draft, "view" will let you look at your guide as it currently stands in draft form, and "publish" will update the guide for everyone to see.
Once you're editing, the same rules apply as when you originally created it. "Save" will save your progress as a draft, "view" will let you look at your guide as it currently stands in draft form, and "publish" will update the guide for everyone to see.

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