Unfortunately, you're out of luck here.
There is inherit
to copy a certain value from a parent to its children, but there is no property the other way round (which would involve another selector to decide which style to revert).
You will have to revert style changes manually:
div { color: green; }
form div { color: red; }
form div div.content { color: green; }
If you have access to the markup, you can add several classes to style precisely what you need:
form div.sub { color: red; }
form div div.content { /* remains green */ }
Edit: The CSS Working Group is up to something:
div.content {
all: revert;
}
No idea, when or if ever this will be implemented by browsers.
Edit 2: As of March 2015 all modern browsers but Safari and IE/Edge have implemented it: https://twitter.com/LeaVerou/status/577390241763467264 (thanks, @Lea Verou!)
Edit 3: default
was renamed to revert
.