Up for discussion:
The userscript I (and others) have written seems to be fairly popular among people who have found it. I’ve had several requests to turn it into a Lemmy theme, but I have been hesitant to do so. Partly because I lack the skill to do it, and partly because it isn’t really a “theme” as such. It doesn’t change the colors or do some of the things a theme typically does. In truth, the script is actually (essentially) theme agnostic - I use it with darkly, but it works with all of the other Lemmy themes without issue (hopefully).
I have no qualms with someone helping turn the CSS stuff into a Lemmy theme (or just taking the bull by the horns and doing it) , but with the changes coming in v.0.18 it may be better to wait.
Anyone have any thoughts? Is this script even appropriate for a Lemmy theme?
for your second point: yes, proper testing would require a theme separate from the defaults.
for your first point - that’s where the work is: replacing only the things that are modified by the script to use the script values in addition to any color theming. It is a big job… One that may not be doable at this time.