I can see how that could be more convenient. I don't see any problems with it other than if the constant was declared more than once. I suppose if it was declared in the given CSS file, then it would override any other declarations. I'll have to think about this a bit more.
Perhaps...
Global values supplied programmatically or via XML are treated as "default" values.
When all CSS is loaded, if there are constants declared in them and not globally, then it uses the first found declaration as the default.
Then, as each CSS file is processed, if it contains a constant declaration which differs from the default, then it overrides the default.
How does that sound?