I, unfortunately, wasn't very clear, in my original comment, that I am making a distinction between theoretical computer science (the topic); and being a computer science major.

Theoretical computer science is generally regarded as a topic in applied mathematics. However, it doesn't follow that, therefore, a computer science major is basically a mathematics major.

Theoretical computer science (the topic) is a component of a degree in computer science. How large of a component it is varies greatly from university to university and, perhaps, has changed over time.