protected static function compute_theme_vars( $settings ) {
    $declarations  = array();
    $custom_values = isset( $settings['custom'] ) ? $settings['custom'] : array();
    $css_vars      = static::flatten_tree( $custom_values );
    foreach ( $css_vars as $key => $value ) {
      $declarations[] = array(
        'name'  => '--wp--custom--' . $key,
        'value' => $value,
      );
    }

    return $declarations;
  }