I came here to post the same observation as OP. Used to be, you’d set default colors in your browser, and those would be used to render all web sites. No need to use a plug-in, override CSS per-site. If you wanted to, you could have the web site render into your own “brick wall” background.

Over time, browsers have ceded more and more control over layout and colors to web developers, and got busy burying/deprecating the browser-side defaults... to the point where user preference is an afterthought now, and almost universally ignored by sites. Now you have to use big hammers like disabling CSS and JavaScript in order to have any say as a user. Nobody likes when your native desktop application's UI ignores your system default colors and forces their own color scheme, yet this is acceptable on the web.

Basically, yes. Not unique css mind you, but a single nicely readable (for me) set of styles for everything. This stoves a bunch of accessibility problems too. Why is this weird?

> You want to be writing the CSS … for every site on the internet that you access?

Yes, I want to set a style, once, and have it used by every site on the internet that I access.

No, just one global stylesheet that all websites obey without breaking. The (dead) dream of the ideal user agent is a program that lets you browse with your preferred color scheme, fonts, font size, etc; websites look just like any other program. Or not, if you prefer. Content is dictated by the authors but appearance is dictated by the user-agent.

The idea of the user agent dictating presentation rather than the content author is kind of dead on today's modern web; while it's certainly possible to set a global stylesheet, doing so creates an incredibly frustrating game of trial-and-error as some websites break while others work fine.

The idea does live on outside the mainstream in ideas like Gemini, Gopher, and a small subset of the Web that focuses on textual content with very conservative progressive enhancement.

I wrote about this previously: https://seirdy.one/2020/11/23/website-best-practices.html

Most websites should be able to work just fine with nothing but HTML. That doesn not mean that they should actually be just HTML (my website certainly isn't); it means that enhancements like CSS and JS should be progressive. Exceptions exist, obviously.

> You want to be writing the CSS for every site on the internet that you access?

I interpreted this as OP being tired of having to rewrite the awful CSS of most websites.