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Figured out how to do this!

Set your default font_face to "SourceCodePro-Light" in your Preferences.sublime-settings

Then edit whatever theme you are changing and set ALL the fontStyle properties to "bold" except for the comment scope.  This turns all the other fonts from light to regular.

I'd love to be able to do more than just bold/italic.   "light" or "lighter" for example.

The new Adobe SourceCodePro fonts come in Black, Bold, ExtraLight, Light, Regular and Semibold. I'd like to view my code in Regular and make the comments show one step lighter using Light.