She [topic-は] rarely sing-not [but-が], tonight [topic-は] sing [quotative-と] think.
滅多に歌わないが ('she rarely sings, but') — 滅多に~ない (めったに~ない) means 'rarely, seldom.' が contrasts with the expectation for tonight. A good example of how は after 今夜 topicalizes the exception.