Thanks, everyone! I will try out what you've uploaded, perhaps it works. If not, it's not too tragic, because "Lemmicks" doesn't have the music theme anyway, so I don't need this tileset in there. If the option is available, I might create something entirely new for Lemmicks, though.
With that being said, do keep in mind that the primary reason old versions are kept available is to allow playing content that relies on it (or hasn't been officially updated yet, even if it should work without issue) and creating new content for it is not recommended. Content should always be created for the current stable version, perhaps keeping in mind any changes that are confirmed (or likely) to be occuring in the near future.
Which translates into: Only use the new formats editor, or, if you stick to 10.13, don't use radiation, slowfreeze, anti-splat pads, or turning teleporters.
And please don't use the ceiling.
Well, I for one have made it public that I am creating
a pack trying to systematically approach gimmicks. This is only possible in 1.43. And, who would have thought, already some players are showing interest in it, even at such an early stage in development! Mostly because they enjoy killing lemmings, I guess
. But perhaps also for some of the other stuff.