So the theme is "Cha-cha-cha Changes?" Four theme answers, all beginning with "CHA?" Sure. Why not?*
As Huygens has remarked in the past, it seems a little odd to have seasonal references off-season, and 17A: December display (CHANUKAHMENORAH) falls into that category. It's not bad fill, it's just a little odd in April. CHAOSTHEORY (24A: Mathematical field that includes the so-called "butterfly effect"), on the other hand, is a favorite subject, so I'm quite happy with that. And CHARLOTTEBRONTE (38A: One of literature's "three sisters") is welcome, too. Coincidentally, I just photographed several first editions of the Bronte sisters, many of which were attributed to their alter-egos, the Bell brothers. Charlotte's was "Currer." Lastly, I suppose, I should note the "Huygens material" clue of the final theme entry CHAINSMOKED (44A: Went from butt to butt?).
In non-fill, I love seeing LAILAALI (62A: Boxer who competed on "Dancing With the Stars") in full. It looks so crazy! I've never heard of SHAN (50D: Young-adult fiction author Darren), and BAAS (53D: Farm cries) isn't the greatest, but on the whole I thought this was a pretty clean grid. I actually like the "quote clue" GETTHIS (22D: "Would you believe …") but maybe only because it reminds me of Maxwell Smart. The paired "Zodiac symbols" were nice (I'm a RAM, myself), and the multiple compounds FANBOY, COPCAR, RIBCAGE and LABRAT were fine with me. Overall, a decent Thursday.
* I feel a bit foolish for having missed this, but the real theme is the difference in the sounds of the "CH"s in the four theme entries. I stand corrected and humbled, and I like this even more now that I realize what they've done.