1. It depends. On your skill but also on the theme you are using. Some are much easier to work with than others. I've never tried working with the CSS on Rounded, so I don't know.
2. Yes, you have to pay yearly.
3. Try it out BEFORE you buy. You can preview your changes (but not save them) in the CSS editor available through your dashboard. Don't spend any money until you're sure you can do what you want.
4. There are lots of useful links to CSS tutorials at the top of the CSS forum. Have a look at those. Be aware that there is no staff support for CSS and there aren't many volunteers on the forums who can help either.