In many cases, you can probably get by with your browser's default Reader Mode.
I'll use uBlock Origin's element zapper on a whole heck of a lot of sites, and you can write default rules for common annoyances as well.
If you get fancy, you can write generalised styles which address specific annoyances, and toggle those on or off on specific websites.
Otherwise, I'd found I'd written a thousand or so custom styles over a couple of years at one point. Most were quite elementary (usually font tweaks or removing headers / social link-litter). A few ... somewhat less elementary. Taught myself a lot about CSS in the process.
To quote Thom Yorke, "You do it to yourself, you do; and that's why it really hurts"
The free download buttons are in the middle of the page. There are also multiple links to the source code at the bottom.
Just use the links in the middle of the page and it will install it for free like every other normal browser extension people use all the time.
https://addons.mozilla.org/en-US/firefox/addon/darkreader/
The firefox extension alone has 1 million installs just from this page, we all installed it for free and have been using it for years. I think asserting that there are only paid options is disingenuous to say the least.
I mainly read HN from an RSS reader, whose web view always shows light mode.