Tim,

Ah, common problem. In the "remote view" in dreamweaver, right click on the css file (or folder that contains your css file).
Select "Set Permissions".
You should get a window that contains check boxes for read, write and execute privileges.
Make sure that the the "owner" has "read" and "write" checked off.
Click "okay".
You should be good to go. Let me now if that helps!
Note: sometimes it will still say "unwritable" in the Joomla back-end, even though the permissions have been updated. At least I experienced that once before.

Another tip: I don't like updating the CSS in Joomla's admin panel. It tends to mess with the file permissions to say nothing of it being hard to find the code you are looking for. If you have not done so, get the Firefox web developer plug-in. You can view exactly which css rules are affecting what on your web page. Then modify as needed in Dreamweaver (or text editor of your choice).