What does exactly theoretical computer science entail?
CS isn't neatly divided into "applied" and "theoretical".  Work that seeks to build systems and solve practical problems tends to get described as "applied"; work that seeks to increase our knowledge of computation tends to get described as "theoretical". Machine learning covers a spectrum that includes both.

If somebody said to me "I'm a theoretical computer scientist", I probably wouldn't assume they did machine learning&nbsp;&ndash; but only because people who do machine learning tend to say "I do machine learning."