From the OP, there's also StyleBot: https://stylebot.dev/
a {
color: blue !important;
text-decoration: underline !important;
text-decoration-style: solid !important;
text-decoration-color: blue !important;
}
(Unfortunately, extensions like Stylus <https://github.com/openstyles/stylus> don’t handle user stylesheet priorities correctly: !important in a user stylesheet should override absolutely all site stylesheets, including inline style attributes. But because of lousy technical limitations, they load stylesheets at the site priority rather than the user priority. But then, what you presented doesn’t conquer a site !important spec anyway. See https://wattenberger.com/blog/css-cascade#origin for a really good explanation of all this stuff.)The better one is called Stylus. The main difference being "Any and all analytics, telemetry, and data-collection have been removed completely. We'd rather not know what you're up to."
I still remember the time when user styles were a first class feature built into all browsers. Hell, that 'C' in CSS - "Cascading" - was always there to allow styles to enhance/override prior styles, including allowing the user to override website's styling. Back before web designers ruined everything by making every web page into its own special snowflake, people thought users would have one or two default CSS sheets to choose from and apply to any webpage, the way we today think about "dark mode".
These days, we have to resort to using browser extensions. A well-known one is Stylus [0][1]. In a way, it's much better than old built-in user styles. But then, it's not built in. Still, it's a nice way to fix up some of the frequently visited websites.
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[0] - https://github.com/openstyles/stylus
[1] - Mind the name, it's "Stylus", not "Stylish" - the latter used to be popular, but then it sold out and become another peace of surveillance capitalism detritus. Stylus is a GPLv3 fork of Stylish with data collection removed.