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I’d say that the “core” should be stuff such as simple styling of most common elements, and the grid you’re already working on.
My personal view is that a framework should not be detailing colours and such, and if they do it’s better to use Less/Sass/Stylus to declare some kind of variables and work from there.
Having a framework to scaffold a project is a great idea (I tend to start 90% of my projects with including typeplate and normalizr, for instance), but then it should only scaffold. Not set fonts, colours, etc, unless it’s to ensure consistency over browsers (IE normalizr is fine, I tend to dislike Bootstrap).