Theoretical CS research having a math undergrad
I thought this question was asked and answered before, but I couldn't find it. In any case, people with a strong math background can do very well in TCS. The only stumbling block might be the breadth of CS material (including things like architecture, compilers and operating systems) that might be required of a CS grad student. 

However there are certain programs (CMU/Georgia Tech/Waterloo) in which TCS is a separate discipline: if you applied to one of these programs, you might be able to to circumvent core CS requirements that you don't have.