Bump. I updated to Mavericks a couple of weeks ago and have just slammed into this issue. The only "workaround" I've found is to click the link for the stylesheet that you've just modified and copy the code from that, but that's an extra step that I shouldn't have to perform when text is right there in front of me.
On top of that, I don't seem to be able to even add an attribute to an existing class anymore - hitting Tab, which used to let you quickly start a new attribute in the rule that you're editing, literally inserts a tab into whatever you're typing. Absurd! There are no tabs in CSS other than for visual organization that's irrelevant in this context. Why would the Inspector allow you to type actual tabs here?!
But what, there's more! While I love the new "+ New Rule" button, even here I'm not able to add attributes - unless I'm missing something, I would love to learn that I am - to the new rule. So if this new rule has six attributes, then it displays in the Inspector as six new rules, each with the same name. Absurd x2.
I really do like some of the changes to the Safari 7 Inspector, but I would gladly give them up in order to eliminate these infuriating UI obstacles. It's not worth it, and it's slowing me WAY the **** down. I've been one of the few Safari-centric web devs out there for many years, but if this doesn't get fixed then I'm going to have to finally make the migration to Chrome, and I won't be coming back if I do so.
Bonus gripe: The two irritations above combine to form one "super irritation"! After you've created a temporary rule to your liking, you can't select it. Period. All you can do is select/copy either the rule's class/ID or its attributes, not both. Brilliant.