Browsers have supported user defined CSS defaults since forever, and plugins like Stylish have been around for a while.

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.