Man, I've got to take a couple of seconds just to check crossings before putting the last letter in. I put StEAm in at 56D: What you do in a sauna (SWEAT). Both are acceptable answers, and they share three letters, so it looked pretty good! Sadly, only one is correct, and not the one I chose to enter. What's funny is that I actually "checked" the crossing at 59A, as it is the revealer for the theme, and my brain, marvelous machine that it is, filled in the last letter for me, without actually seeing it fully.
And now, of course, looking over the puzzle again, I see that 4D: Makes furious (STEAMS) would invalidate doubly my chosen answer.
Anyway, today's puzzle is a reasonable Monday. The theme is phrases which contain three letters in a row. KLMAIRLINES will certainly appeal to certain core members of this blog. ABCANCHOR is fine, and XYZAFFAIR is definitely a thing, although its day is long past, such that I had to put it together primarily from the theme.
1A: Alternatives to PCs (MACS) is only okay, if only because, of course, it should be the other way around. Macs are the primary choice, and PCs are what you use when you can't get a Mac. So I'll give it a C-. My favorite clue-answer pair is 3D: Middle of 100,000? (COMMA). Otherwise, all the clues are straightforward.
Didn't like RELEE, ANAT, ISTO, EELY. Otherwise it's fine.