This is all I could find for Mac Lemmings. Looks about the same to me.

I am unsure if the Mac Graphics would conform to the High Rez Mode considering they're the highest detailed graphics for any version of Lemmings (not counting the PSP version) I'm in a position to test it considering I have not tooled with HR mode or had interest in it.

If its just a color palette for the Lemmings however, recoloring is a thing you can do with nxmi theme files

Not sure how likely it is for an in-game option to be added, but the sprites are just PNG files, nothing at all is stopping you (or anyone else who wants to do so) from modifying them to have the look you want. NL is very moddable like that. You could even likely use a direct rip from the Mac version for the sprites that exist there, though you'd need to make custom ones for the new skills.

I remember making a (non-trivial) one-second level as a proof of concept once. It's probably still floating around in the older posts somewhere.

The minimum setting should either be one or zero (in the latter case, with zero being interpreted as "no time limit"). This is not a situation where you have to choose some arbitrary limit (unlike, say, maximum level size where there isn't - within reason - any specific exact cutoff point where it won't work beyond), there is a clear logical line here - we can't meaningfully have negatives, and we can't have literal zero (but can have infinite, and could treat zero as a shorthand for that). Thus, minimum of either 0 or 1. Whereas for upper limit, there's the consideration of "what actually fits on the in-game display" as a good reason for 99:59 being the maximum finite value.

I don't remember a specific reason. (But I don't know all NL design history by heart either.)

Pre-placed lemmings will already start walking before the hatch opens. You can build a meaningful level with a 2-second time limit.

It's harder to make a 1-second level because lemmings take 9-10 physics updates to exit, and they'll score only at the end of exiting. But it's theoretically possible to have meaningful play in the remaining 7-8 updates. If you allow 2 seconds, you should allow 1 second.

-- Simon

Pretty sure this one is by design; it was probably decided at some point that the Editor should have a minimum value, and that value should be 10 seconds.

What may be more appropriate is that there is some non-zero value for the time limit, be it infinite time or at least 1 second.

I'll get this sorted for the next update.

Emoji testing: 😀
I remember the Macintosh-styled lemmings that have eyes, light-tan sleeves and tan hands at the end of the sleeves, and light-tan socks. One day they will be added into the high-res lemmings options (graphics modes 1 (Windows 95-styled) and 2 (Macintosh-styled)).
             Macintosh

I've created - as a training exercise - a small elementary level which resolves perfectly in exactly 9.00 s (0:09. 00), with a Spawn Rate locked at 99, a Time Limit legitimately set at 9 s and 6 lemmings out of 6 saved successfully.

When I validate this level created in SLXEditor 2.8.8 (File/Validate Level), I get the following message: "Not enough time: Only 9seconds available".
If I set the Time Limit to 10 s, when I validate this level, I get the following message: "No issues found".

There seems to be a minimum duration (10 s) for the Time Limit below which you can't go without getting the error message indicated, although the level has been successfully solved.

Is this by design? If so, why? Or is it a bug?

You have my condolences, losing a pet is never easy.

You've noticed Cookie's changes in behavior. That shows your good relationship.

Take good care of her now. 16 years is a proud age for a cat. All the best to both of you!

-- Simon