redwerks

This plugin allows posts and pages to have individual custom backgrounds. A full-body background that shows up on the post page itself can be specified. As well as background that just covers the content area and shows up on all places the post is visible.

Post Backgrounds

Allows posts and pages to have individual custom backgrounds. Both as body backgrounds and as content backgrounds.

ishitaka

XO Post Background is a plugin to set background image and color for each post.
Background image details (size, position, etc.) and background color can only be set in WordPress 5.7 or higher block editors.

XO Post Background

XO Post Background is a plugin to set background image and color for each post.

Post Backgrounds vs XO Post Background