WP_Customize_Manager::_sanitize_background_setting PHP Method

    public function _sanitize_background_setting($value, $setting)
    {
        if ('background_repeat' === $setting->id) {
            if (!in_array($value, array('repeat-x', 'repeat-y', 'repeat', 'no-repeat'))) {
                return new WP_Error('invalid_value', __('Invalid value for background repeat.'));
            }
        } elseif ('background_attachment' === $setting->id) {
            if (!in_array($value, array('fixed', 'scroll'))) {
                return new WP_Error('invalid_value', __('Invalid value for background attachment.'));
            }
        } elseif ('background_position_x' === $setting->id) {
            if (!in_array($value, array('left', 'center', 'right'), true)) {
                return new WP_Error('invalid_value', __('Invalid value for background position X.'));
            }
        } elseif ('background_position_y' === $setting->id) {
            if (!in_array($value, array('top', 'center', 'bottom'), true)) {
                return new WP_Error('invalid_value', __('Invalid value for background position Y.'));
            }
        } elseif ('background_size' === $setting->id) {
            if (!in_array($value, array('auto', 'contain', 'cover'), true)) {
                return new WP_Error('invalid_value', __('Invalid value for background size.'));
            }
        } elseif ('background_preset' === $setting->id) {
            if (!in_array($value, array('default', 'fill', 'fit', 'repeat', 'custom'), true)) {
                return new WP_Error('invalid_value', __('Invalid value for background size.'));
            }
        } elseif ('background_image' === $setting->id || 'background_image_thumb' === $setting->id) {
            $value = empty($value) ? '' : esc_url_raw($value);
        } else {
            return new WP_Error('unrecognized_setting', __('Unrecognized background setting.'));
        }
        return $value;
    }