Is there a web app to edit CSS on any website, and share the result?

Not that i've seen, but i've wanted exactly this before, especially to show clients possible tweaks to a site.

Have a look at CSSEdit. I've never actually used it, but I think it fits your description.

Edit: Not sure about sharing results though

I'm a PC. :(

Chrome is so decent at modifying CSS & previewing the changes, seems like there could be a (simple?) extension that would track changes you make and give you a shareable preview link. Right? Are there other complexities I'm missing?

Do you have a contact email? I can show you a quick preview of something I have...it's very rudimentary, but it works.

Not the OP, but my profile has notes on where to find my email address, if that offer is open to more than just the OP.

Added my email's to my profile. Thanks for willingness to share!