Topics from theoretical computer science suitable for a bachelor (undergrate) thesis?
As a bachelor student that is writing his thesis in that field I can guarantee you there are a lot of topics suitable for a bachelors degree. 
Personally, I am working on graphs and some specific algorithmic problems over those.
I suggest you ask your professors if they have some topics for you. 
At my university usually professors have a list of "problems" and we can either choose between one of those or propose our own topic.
Obviously because it is "just" a bachelor thesis you'll have to be specific and concentrate on some smaller problems e.g. 


Formal verifiation of smaller programs in some specific language (like haskel)
Finding some polynomial algorithm to a specific problem that hasn't been researched too much. (Also, if you can't find anything new, explaining why the problem is "complex" can also be a good plan b)
Testing of specific cryptographic protocols under specific assumptions (allowing the attacker to have some specific information etc..)