Thematically it makes sense for the later chapters to have a more uplifting leitmotif. There are still bits of despair here and there, but post ch 24 Commander is a lot more competent, has seen some shit, has made a solid network of allies both within and outside the Ark, and has made reclaiming the surface an actual viable goal (if ever so incremental) instead of a pie in the sky empty hope. Ch 30 theme really hits this culmination of all his character growth we have seen in the "prologue."