Overlap between theory and systems fields in CS
I can see a lot of overlap in the field of Cloud Computing or Distributed Computing
- being that it relies heavily on process scheduling and parallel processing on the operating systems side and optimization algorithms to both traverse data across multiple nodes (servers) and optimize hardware resources on the network grid. Graph theory and NP issues come up often when trying to reconcile issues concerning "Big Data" within the distributed computing space. 

The software engineering CS component arises when developing designs for new operating system based applications like system monitors that help stabilize and secure distributed computing systems.  

From this example, one can see much of the overlapping of said disciplines that go into Cloud Computing.