Emmerich Kálmán’s Gräfin Mariza is celebrating her 90th birthday this year, but just like my slightly younger great-aunts, who introduced me to this operetta at the tender age of five, she is far from ready for a care home. Thomas Enzinger and set/costume designer Toto have come up with another imaginative response to the question of why and how the bygone genre of operetta should be produced today.