How to transition into a PhD in Theoretical Physics?
Your situation is not much different from other people seeking to pursue condensed matter theory.  There's a lot of overlap between theoretical chemistry and condensed matter physics, so there's no real scientific barriers.  The regulations of individual universities may matter, but we can't help you there.

People who brand themselves as chemists tend to pick RSC or ACS published journals.  People who brand themselves as physicists tend to pick APS or AIP published journals.  Otherwise, it can sometimes be hard to tell the difference.