The whole point of splitting them up is to reduce the amount of diffs needed to upgrade the templates. With both a custom.css and core.css file, you can get away with not having to do the diff at all, while keeping your templates up to date. Of course this comes at the expense of an extra file load, but remember that css files are cached, so it isn't such an expensive trade off.

diffs will be generated like the template diffs we have been providing with our recent releases.

Note that we're currently looking into the different possibilities in making different styles for each template easier, so there might be some changes in how the css is laid out in the RC.

Adrian