The theme files are in the qa-theme folder under the theme name (i.e. qa-theme/Snow for Snow theme). See the docs for more info.
However, the simplest solution is to go to Admin > Layout, tick "Custom HTML in <head> section of every page" and put your Analytics code in there.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.
July 15, 2020: Q2A 1.8.5 released!