code: func (b *Box) SetBorderAttributes(attr tcell.AttrMask) *Box {
  	b.borderAttributes = attr
  	return b
  }
docs: // SetBorderAttributes sets the border's style attributes. You can combine
  // different attributes using bitmask operations:
  //
  //   box.SetBorderAttributes(tcell.AttrUnderline | tcell.AttrBold)