"dark" and "light" are the only valid options and you have to have at least one of them. If you do have at least one of them, invalid names for the others will be ignored.
Try things like this instead:
"wallpaper": [ { "id": "set1", "title": "", "dark": { "image": "wallpapers/set1/dark.png", "text_color": "#ffffff", "text_shadow": "#ffffff" }, "light": { "image": "wallpapers/set1/light.png", "text_color": "#ffffff", "text_shadow": "#ffffff" } }, { "id": "set2", "title": "", "dark": { "image": "wallpapers/set2/dark.png", "text_color": "#ffffff", "text_shadow": "#ffffff" }, "light": { "image": "wallpapers/set2/light.png", "text_color": "#ffffff", "text_shadow": "#ffffff" } }, ]When you set the title, that title will show when hovering over the wallpaper in the wallpaper selector slider.