Example bad:
#main {
padding:0;
margin: 10px auto;
}
Example good:
#main {padding:0;margin:10px auto;}
<
Here's a one-liner:
:%s/{\_.\{-}}/\=substitute(submatch(0), '\n', '', 'g')/
\_. matches any character, including a newline, and \{-} is the non-greedy version of *, so {\_.\{-}} matches everything between a matching pair of curly braces, inclusive.
The \= allows you to substitute the result of a vim expression, which we here use to strip out all the newlines '\n' from the matched text (in submatch(0)) using the substitute() function.
The inverse (converting the one-line version to multi-line) can also be done as a one liner:
:%s/{\_.\{-}}/\=substitute(submatch(0), '[{;]', '\0\r', 'g')/