Browsers should be making this a prominent and first-class feature.
Users should be able to quickly and easily set their default styles for various elements and choose at what precedence level they apply.
The practice of users customizing their CSS isn't prevalent because modern sites are incredibly complex and the average user doesn't want (or know how) to spend the time necessary to reverse engineer a site's layout and rewrite its CSS - there is no "quick and easy" way to do that for any but the simplest sites. Just consider how often professional developers complain about CSS, and they get paid to put up with it.
It looks like Chrome removed these several years ago:
https://news.ycombinator.com/item?id=7329855
Plugins are a thing but why should you need a plugin for something that is a core part of the CSS specification?
https://www.w3.org/TR/CSS2/cascade.html#cascade
> the average user
This is (mostly) about accessibility. Even if the average user does not need it, some do. And those that do would be quite glad to have it easily.
> there is no "quick and easy" way to do that for any but the simplest sites
Just because user stylesheets have been ignored for many years doesn't mean they shouldn't be acknowledged. Design your site with them in mind.
Actually, yes:
https://www.w3.org/TR/CSS2/cascade.html#cascade
User stylesheets are supposed to be applied at a precedence level below the page author's style, so any time the page author did not specify a style, the user's style can be used as default.
Browsers should have made this an easily accessible feature, but they seem to instead like to remove access to it. Think "Developer tools window", but persistent.