Ahhhh Monday. The return of the simple puzzle. Easy and educational.

30A Ancient carver of stone heads in Mesoamerica: OLMEC. Olmec or Ol’ Mec? To the googles!

They were an entire civilization independent of the Mayas. Worth a read: https://www.mexicanist.com/l/olmec-heads/

Here’s something glaringly sloppy in this crossword:
42D H2O, south of the border: AGUA
47D Light blue shades: AQUAS

But 53D “Huh, funny running into you!”: OHHI was funny because I’m a big fan of the film The Room.

The theme clues of this puzzle are related to 57A The terrible twos, e.g. (one hopes!) … or the start of 17-, 22-, 36- or 45-Across?
JUSTAPHASE
17A Where to go for a fill-up: (GAS)STATION
22A Firm place to plant your feet: (SOLID)GROUND
36A Viewing options popularized in the 1990s: (PLASMA)SCREENTVS
45A Cash or stocks, e.g.: (LIQUID)ASSETS

Pretty good.