wp-includes/theme.php:3949Creates the initial theme features when the 'setup_theme' action is fired.
function create_initial_theme_features() { register_theme_feature( 'align-wide', array( 'description' => __( 'Whether theme opts in to wide alignment CSS class.' ), 'show_in_rest' => true, ) ); register_theme_feature( 'automatic-feed-links', array( 'description' => __( 'Whether posts and comments RSS feed links are added to head.' ), 'show_in_rest' => true, ) ); register_theme_feature( 'block-templates', array( 'description' => __( 'Whether a theme uses block-based templates.' ), 'show_in_rest' => true, ) ); register_theme_feature( 'block-template-parts', array( 'description' => __( 'Whether a theme uses block-based template parts.' ), 'show_in_rest' => true, ) ); register_theme_feature( 'custom-background', array( 'description' => __( 'Custom background if defined by the theme.' ), 'type' => 'object', 'show_in_rest' => array( 'schema' => array( 'properties' => array( 'default-image' => array( 'type' => 'string', 'format' => 'uri', ), 'default-preset' => array( 'type' => 'string', 'enum' => array( 'default', 'fill', 'fit', 'repeat', 'custom', ), ), 'default-position-x' => array( 'type' => 'string', 'enum' => array( 'left', 'center', 'right', ), ), 'default-position-y' => array( 'type' => 'string', 'enum' => array( 'left', 'center', 'right', ), ), 'default-size' => array( 'type' => 'string', 'enum' => array( 'auto', 'contain', 'cover', ), ), 'default-repeat' => array( 'type' => 'string', 'enum' => array( 'repeat-x', 'repeat-y',Introduced in 5.5.0. 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.