Theoretical Computer Science Stack Exchange (“CSTheory.SE”) is for
research-level questions in theoretical computer science
My eyeball evaluation is that about half the questions so far are about theoretical computer science, and I don't know that any of them are research-level.
I don't want to declare that research-level questions in TCS are off-topic here. While this would theoretically make CSTheory.SE a subset of CS.SE, I do not expect many CSTheory-type questions to be asked here, because the CSTheory community is better suited for them. I don't think we have to make any effort to differentiate this site from CSTheory, it'll happen naturally.