Custom CSS risks being scope creep, IMO. A dark mode with options of a standard dark and a pure black dark would likely account for 99% of use anyways.
Myself and others with astigmatism literally cannot use "dark mode", tastefully done it's fine tho, talking stuff like Nord theme, monkeytype.com etc as long as it's not straight black and white.
Classifying and prioritising those, and perhaps sharing some of the list, would be a good first approach even if you don't go further.
It'd also be helpful to request rationales for why different modes are requested.
My own major gripes about HN:
- Default font is too small. Screen sizes, resolutions, and dot pitches have evolved dramatically since 2007, in multiple ways. One Size Does Not Fit All. (FIMS -- fixed in my style)
- Default colour scheme is just plain hard to read, all the more so on e-ink (which I use for much of my reading). (FIMS)
- Voting controls are too hard to hit, and easy to confuse. (FIMS)
- For those who want that sort of thing: no dark mode. (Fixed in my other style.)
- Text dialogue too small. (FIMS)
- Various navigation / mode elements hard to distinguish. (FIMS)
- Providing additional colour context to additional screen elements. (Fixed in my personal style, not on the shared style.)
Going back to the default HN style always feels exceptionally jarring.
Outside CSS tweaks, I'd prefer at least a proper quoting/blockquote style. HN doesn't need to go full Markdown (though That Would Be Nice), but this is one key lacking feature.