What's the issue?

Site background images in WordPress pre-block editor

in this issue a "site background image" refer to an image set via the background-image CSS property to the body tag.

WordPress themes have long been able to set site background images and other background properties such as size and repeat (on the HTML body element) either by the Customizer UI or, more directly, using custom-background theme support. For example:

add_theme_support(
    'custom-background',
    array(
        // As well as default image, there's default-position-x, default-position-y, default-size, default-repeat, default-attachment
        'default-image' => get_stylesheet_directory_uri() . '/img/bird.jpg',
    )
);

Or like this:

add_theme_support( 'custom-background' );
// As well as background_image, there's background_preset, background_size, background_position_x, background_position_y
set_theme_mod( 'background_image',  get_stylesheet_directory_uri() . '/img/bird.jpg' );

Enabling background support adds a link to the Customizer in the admin menu. Users can add/edit the background image using Customizer controls:

Link in sidebar	Customizer controls

Site background images in block themes/block editor

Since #59354 and #59454, users can add site background images via theme.json or global styles in the site editor.

However, if a site background image is set via theme support/mods, it will overwrite any theme.json or global styles image set in the site editor because of CSS specificity, that is body.custom-background will trump :where(body).

What is the question? Or your proposed solution?

The main question is whether Gutenberg should take values set by theme support/mods into account by:

• using any background- values from theme support/mods as defaults for top-level background property values.
• overwriting theme support/mod values with theme.json/global styles values, perhaps via a higher CSS specificity or preventing WordPress from outputting the custom-background-css style tag
• ??

Related discussion: #60578 (comment)