There's nothing here that would stop you from still doing that.
But if we're talking about practicality... I think far more people would get a benefit from improved defaults (as long as they're not breaking existing sites).
While I've modified the user styles for my chrome profile as a part of being a web developer, I'm not aware of a single non-developer acquaintance who has (literally - none). Honestly, not even many of the devs I've worked with have done it.
It's just not all that relevant when the defaults are so bad that all the sites are overriding them anyways. Makes it very hard to apply a set of user styles that's sane, so no one does.
Make the defaults better, and maybe you can end up back there.
My browsing experience should not be determined by an artist or product designer 1000 miles away from me in an office somewhere.
My language skills and time pressure fails me here, just wanted to express general "thanks folks" for your reactions as well. Seeing that someone shares similar point of view on this topic made me surprisingly very happy, so thank you again.