I understand... looking for answers can be frustrating.
To address your point as directly as I can, I don't believe that wordpress logs admin activity. I hope that for your sake I'm wrong, but I am yet to see any evidence of it.
Short of trawling your server's access logs looking for hits on the theme page from known IP addresses, I don't see how you're going to get joy here.
The reason I didn't say that to begin with was that I don't believe it to be a solution in any sense of the word... and also because if you could manage trawling your webserver's access logs, you probably would have thought of that by now.
In my view, I just posted the least useful thing I've said in this thread, but it was as direct as I could be towards addressing your question.
Lets hope someone else can be more useful to you.