code: function TEXT_STYLE(mode: number, fontSize: number): string {
  
    if (mode == DisplayMode.LOOSE) {
      return  `${fontSize}px 'Helvetica Neue', Helvetica, Arial, sans-serif`;
    } else if (mode == DisplayMode.TIGHT) {
      return `bold ${fontSize}px 'Helvetica Neue', Helvetica, Arial, sans-serif`;
    } else {
      return `${fontSize}px`;
    }
  }
docs: Computes styles for base pairs which are rendered as letters.
  
  @param mode DisplayMode to render text.
  @param fontSize font size.