Source

File: wp-includes/theme.php

function _remove_theme_support( $feature ) {
	global $_wp_theme_features;

	switch ( $feature ) {
		case 'custom-header-uploads':
			if ( ! isset( $_wp_theme_features['custom-header'] ) ) {
				return false;
			}
			add_theme_support( 'custom-header', array( 'uploads' => false ) );
			return; // Do not continue - custom-header-uploads no longer exists.
	}

	if ( ! isset( $_wp_theme_features[ $feature ] ) ) {
		return false;
	}

	switch ( $feature ) {
		case 'custom-header':
			if ( ! did_action( 'wp_loaded' ) ) {
				break;
			}
			$support = get_theme_support( 'custom-header' );
			if ( isset( $support[0]['wp-head-callback'] ) ) {
				remove_action( 'wp_head', $support[0]['wp-head-callback'] );
			}
			if ( isset( $GLOBALS['custom_image_header'] ) ) {
				remove_action( 'admin_menu', array( $GLOBALS['custom_image_header'], 'init' ) );
				unset( $GLOBALS['custom_image_header'] );
			}
			break;

		case 'custom-background':
			if ( ! did_action( 'wp_loaded' ) ) {
				break;
			}
			$support = get_theme_support( 'custom-background' );
			if ( isset( $support[0]['wp-head-callback'] ) ) {
				remove_action( 'wp_head', $support[0]['wp-head-callback'] );
			}
			remove_action( 'admin_menu', array( $GLOBALS['custom_background'], 'init' ) );
			unset( $GLOBALS['custom_background'] );
			break;
	}

	unset( $_wp_theme_features[ $feature ] );

	return true;
}