Use of first person in a PhD Thesis
I generally avoid "I" in scientific texts altogether, though some authors are in fact using it if they are the sole author. I can't remember seeing it in a thesis though. In texts with a sole author, I usually understand "we" as meaning the author and the reader, and I'd suggest that it's fine to use it in places where it can have that meaning. For example something like "When substituting a by b, we get ..."

A generally useful advice would be to read into some of the theses written in your group, department, and university (in decreasing relevance), and see whether there is a common pattern.