Awesome - this works reliably and does not spy on me like stylus and stylish do. I would love to see it expanded to manage styles by url in the future, that's why only four stars ;)

The only thing missing is sync between firefoxes.
One of the must haves if you take browsing seriously. My wikipedia.org customizing


.mw-body-content p {
text-align: justify;
hyphens: auto;
max-width: 70ch;
text-wrap: pretty;
}

Throws all kinds of errors for no reason, even with basic elements like #header.
It doesn't have any formatting aids either, though I suppose if all you care about is size that's not a dealbreaker.