Are there any topics in theoretical CS that are more about pure math?
Here are three more fields that fit your criteria.


Category theory. This is clearly interesting to most pure maths
fields, but also has been very influential in the theory of
(functional, sequential) programming languages.
Logic, particularly proof theory. The connections with computer
science are too many to name, but logic is not only a rich field of
pure mathematical, but the foundation of mathematics.
Number theory, the "queen of mathematics", which was deemed to
be devoid of applications ... until cryptography came along.