Developer for Ace here.

The syntax highlighting rules for Ace were largely derived from tmlanguage files. This allows us to pretty easily import existing definitions. For example, Google had a tmlanguahe file for Dart that I was able to translate in less than an hour.

Could you describe more about what you like about CM's highlight rules? I admit I've never really looked into them. But I'm guessing by "show local variables in a different color" there's some parsing going on?