One of the central themes in Twelfth Night is strength of love, or fickleness. A second theme is even the arbitrariness of love. Since love is always arbitrary, people throughout the ages will always have a tendency to be fickle; therefore, fickleness and even the arbitrariness of love will always be relevant for any time period, and will especially have modern relevance.
We especially see the theme of the arbitrariness of love disclosed in the scene in which Olivia meets Viola as Cesario for the first time. Olivia tells Cesario that she cannot possibly return Duke Orsino's love, even though she knows he is a virtuous, noble, intelligent, and even brave man, as we see in her lines:
I cannot love him:
Yet I suppose him virtuous, know him noble,
Of great estate, of fresh and stainless youth;
In voices well divulged, free, learn'd, and valiant. (I.v.242-245)
In other words, in these lines, Olivia is describing Orsino as an excellent catch, yet for arbitrary reasons, she feels she absolutely cannot love him. Instead, she falls for Cesario who is so obviously not a real man that he is described as being too under developed to be a man yet too old to be a boy. Viola as Cesario is even a bit cruel to Olivia in her open judgements of Olivia, calling her overly proud and the cruelest of women. Yet, again, for arbitrary reasons, Olivia prefers Cesario over Orsino.
The theme of the fickleness of love is disclosed in several speeches, especially the conversation between Cesario and Orsino in Act 2, Scene 4 in which Orsino admits that men can be very fickle. However, the theme of fickleness is further proved by the final events of the play. Olivia very quickly switches from loving Cesario to loving Sebastian, simply because she learns Cesario is really a woman, Viola. In addition, Orsino quickly switches from loving Olivia to loving Viola, simply because he learns she is a woman who loves him just as much as he had loved Olivia. While these are correct choices, the suddenness of the choices certainly do show the fickle nature of love. And since the fact that love can be both fickle and arbitrary are enduring truths, we see very easily how the play is relevant for all eras, even today.