How thoroughly do referees review papers submitted to theoretical computer science conferences?
It varies.  As a first year PhD student I was asked to review a theory paper, even though I had almost no background in the field.  The professor explicitly said I only had to read the first half of the paper, and I only had to tell him if it looked interesting and correct.

At the same time I'm sure many reviewers do a diligent job and don't outsource their reviewing to inexperienced graduate students.

Related: http://www.phdcomics.com/comics/archive.php?comicid=1760