FWIW, one redeeming characteristic of SaaS web apps is that most of the time, you can fix up their crap UIs with liberal application of CSS (via e.g. Stylus extension). It's something I find myself doing increasingly often - usually not to make things look different, but to undo some dumb design decisions like low density, whitespace-rich presentation, or hidden video controls.
Unfortunately, this does not give the ability to do system-wide restyling of the kind _trampeltier mentioned. Despite professions to the contrary, almost nothing whatsoever on the web is in any way semantic. Each website is its own assortment of CSS hacks hooked up to a tree of randomly nested, randomly named divs. You can't just write one style that says "please stop with the whitespace around media boxes", you have to unbreak each site individually, because a "media box" isn't really a website-independent thing, and neither is a button.
An alternative to the Stylus extension that the parent comment mentions is the Stylish extension, which has a "marketplace" almost exactly what you're looking for. You don't even need the Stylish extension as each theme is also offered as a userscript.
Yeah, there's that; I wasn't sure about affiliation status between userstyles.org and Stylish.
FWIW, I still recommend installing Stylus instead of Stylish - the difference is that at some point, Stylish changed owner and acquired analytics; Stylus is a fork that's free of privacy-invading antifeatures.
Unfortunately, this does not give the ability to do system-wide restyling of the kind _trampeltier mentioned. Despite professions to the contrary, almost nothing whatsoever on the web is in any way semantic. Each website is its own assortment of CSS hacks hooked up to a tree of randomly nested, randomly named divs. You can't just write one style that says "please stop with the whitespace around media boxes", you have to unbreak each site individually, because a "media box" isn't really a website-independent thing, and neither is a button.