Personal collection of styles and article templates
Article Templates: https://github.com/iamromeo/worldanvil-templates/tree/master/article-templates/base-template
Style rules: https://www.worldanvil.com/w/WorldAnvilCodex/a/css
World Anvil Themes: https://www.worldanvil.com/w/WorldAnvilCodex/a/theme-references
If you just want to swap the background image to another one of world anvil's hosted backgrounds, here's a selection.
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/galaxyanvil/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/scifi/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/notebook/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/rust/images/PaperTexture.png
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/stars/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/strangerworlds/images/background.jpg
URL: https://www.worldanvil.com/uploads/images/7059a770e7367fed76faeabc0910c001.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/trek/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/forest/images/page-background.png
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/deadhouse/images/background.jpg
URL: https://wa-cdn.nyc3.cdn.digitaloceanspaces.com/assets/themes/darkly/images/articlebackground.png
URL: https://www.worldanvil.com/themes/deadhouse/images/background.jpg