What are the recents topics of research that combine both systems research and theoretical computer science? I am an undergraduate majoring in Computer Science and Mathematics. 
For my undergraduate research, I would like to work on something that lies in the overlap between systems research and theoretical computer science. Something that could satisfy this requirement is cache-oblivious algorithms because it is about the analysis and design of algorithms without completely ignoring how actual systems work (memory hierarchy and so on). But there could be many other fields in systems research which involve a lot of theory, or vice versa.

One of my seniors suggested distributed systems - he seemed to indicate that things like Paxos and Raft have a lot of underlying theory, but I haven't taken the time to dig into it.

You could also suggest some field in systems research which can gain a lot from some rigorous mathematical foundations and analysis. A senior will be going to graduate school at Johns Hopkins University to do a PhD on the math behind deep learning. So maybe I could try something like "the math behind --some systems research field--".

Another example I found is "Distributed Computing using Combinatorial Topology". 

TL;DR: What fields/topics of research lie in the intersection of theoretical computer science and systems research?
This might not be what you mean by "theoretical computer science", but there's some rich interaction between Programming Languages Theory and Distributed systems going on with Session Types. They basically are a way to use type systems to specify communication protocols, increasing safety in distributed languages.

For example, The Imperial Mobility Group has many publications in session types, this should at least get you started on finding research in the area.