progressbar.go
func (b *ProgressBar) SecondaryColors() (term.Attribute, term.Attribute) {
	return b.emptyFg, b.emptyBg
}
func (b *ProgressBar) SecondaryColors() (term.Attribute, term.Attribute) {
	return b.emptyFg, b.emptyBg
}
// SecondaryColors returns text and background colors for empty
// part of the ProgressBar