Computes styles for base pairs which are rendered as letters.

@param mode DisplayMode to render text.
@param fontSize font size.
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`;
  }
}