code: func (s Style) Background(c Color) Style {
  	if c == ColorDefault {
  		return (s &^ (0x1ffffff | styleBgSet))
  	}
  	return (s &^ (0x1ffffff)) | (Style(c) & 0x1ffffff) | styleBgSet
  }
docs: // Background returns a new style based on s, with the background color set
  // as requested.  ColorDefault can be used to select the global default.