progressbar.go
func (b *ProgressBar) SetSecondaryColors(fg, bg term.Attribute) {
	b.emptyFg, b.emptyBg = fg, bg
}
func (b *ProgressBar) SetSecondaryColors(fg, bg term.Attribute) {
	b.emptyFg, b.emptyBg = fg, bg
}
// SetSecondaryColors sets new text and background colors for
// empty part of the ProgressBar