Disabling "modal sheets" in OS X
I highly doubt you can change the appearance of sheets, even through undocumented means – that's been a core part of OS X since 10.0.

That said, I think there's a way to effectively avoid the problem you're having: in System Preferences, go to General and uncheck the box for "Ask to keep changes when closing documents".



Now, apps that support auto-save (like TextEdit) won't bug you about unsaved changes when they quit – they'll simply save the changes in the background automatically, then restore them when you next launch the app.

(Note: I think the above option is only available on Mountain Lion).