I use the Stylus extension for Chrome (also available on Firefox) to write user styles for sites, mostly to improve the legibility of text, but for sites like HN and Reddit that I browse a lot I've customised their look and feel extensively to match my tastes. Creating a custom style for a site just requires you to click the Stylus button in the toolbar, click on the site name at the bottom of the menu to create a new style and open the editor in a new tab and then add your CSS and hit save. You can also search for existing styles for your current website that have been published online.

You can have one style apply to multiple domains, I have a simple one for making text black and setting the font size and line spacing which works on a dozen sites or so. You can even use SCSS or Stylus instead of plain CSS if you want.

https://chrome.google.com/webstore/detail/stylus/clngdbkpkpe...

I don't do this personally, so I'm fuzzy on the details. I know of no way to make this work in general (how would you do that?) across sites, but Wikipedia pages follow a consistent format, so that shouldn't be too hard. Furthermore, someone[1] here on HN showed how you can control how the theme works.

My point: if you care that deeply about how Wikipedia in particular is laid out, you likely have the tools to force it to work that way. Meanwhile, the rest of us plebs get a better (from a typographical standpoint, not necessarily a case-by-case basis) default.

[1]: https://news.ycombinator.com/item?id=34431533

If you create an account on Wikipedia then you can go into preferences and define custom CSS styles.