Dropbox will not inevitably cause disaster - but you do need to exercise care, as that thread illustrates.
Are you wanting to share RW 6 Project files?
What are you changing; what is your client changing?
When you say you ‘save’ the Theme (Endeavor) in DropBox, what exactly do you mean?
There will be greater functionality in the upcoming RW 7 for supporting (some such) file synching across computers.
Until then, the Themes should reside in ~/Library/Containers/com.realmacsoftware.rapidweaver6/Data/Library/Application Support/RapidWeaver/Themes
Where and how did you modify the CSS?
I think I’d suggest that you only change the CSS from within the Master Style of one project in RW 6.
You can certainly save your changes as a distinct style and - after revealing the Styles in the Finder (~/Library/Containers/com.realmacsoftware.rapidweaver6/Data/Library/Application Support/RapidWeaver/Theme Styles) - send your client the resulting file (a group of settings) for him to load into his Project. Provided he first has both the Endeavor Theme and an identical copy of the Project file, which you could certainly exchange, bearing in mind the above caveats re DropBox, via DropBox.
Actively trying to synch and share Themes saved in DropBox on the fly as it seems you may have been doing is definitely not to be advised, I’m sorry.
You might want to get everything stable on one version (your machine), then send him everything (Project, Theme (licencing permitting) and Theme Styles) working as you want them to. He too ought to be able to publish at that point without problems.
RW 6, though, is not really designed as a collaborative tool in the way I think you may have been attempting.
An alternative is to use a Content Management System like Armadillo: you set it all up and your client can then easily change the actual text and images via his browser.