Actually, in the US, you have a lot of time after entering a doctoral program with a bachelors to choose a specialty. The first years are advanced coursework leading to comprehensive exams. You get to look at both areas of specialization and faculty members who might advise you. 

Whether a university is more interested in theoretical CS or hot topics like AI depends on the faculty and also on which of the faculty are looking for doctoral advisees. 

I wouldn't be too concerned, but it would probably be good to keep your options open for the moment and project some flexibility in a statement of purpose. Saying that your current interests are in more theoretical aspects is almost certainly fine. 

Note that acceptance in the US is broad based. A single thing, especially a statement of interest in theory, is likely to have very little weight compared to things like GPA, letters of recommendation, specific courses, and such. 

Don't overthink it.