wp-includes/theme.php:1840Retrieves background image for custom background.
stringfunction get_background_image() { return get_theme_mod( 'background_image', get_theme_support( 'custom-background', 'default-image' ) );}Introduced in 3.0.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/theme.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.