I think my main issue was font size and lack of whitespace. I'm typically reading 10-12pt fonts in docs, code, and body content on web pages. HN by default is close to that. I think I'm therefore used to that and find it the easiest size to scan content in – I'm not reading every word I'm skimming through quickly a lot of the time.
That's also where whitespace can help. When skimming I found the page to be a mess of text with little separation. More whitespace, more contrast between different types of text, etc, could do a lot to make the page more easily readable.
One thing that I do find annoying with HN is the long text lines. I'd love to be able to wrap paragraphs at a relatively small width – this typically aids reading and comprehension, and it's why most publishing websites limit content width even on wide monitors. This could also be related to the layout issues I have with this extension – My browser window is currently ~2000x1900px, and it seems the content expands continually with no limit. Maybe there could be a limit to aid readability?
I find content much harder to read in dark mode and don't use it. It strains my eyes with the contrast and I find it harder to focus. I appreciate some people like it but I think it's a personal thing and important to have both options.
Can I ask what specifically was hard to parse ?
I can also provide preferences for any recommendations you may have, that can benefit one. Will add the font-size change for example, as commented elsewhere.
Also, dark mode is more suitable for low light conditions, and does make reading much easier.