I've recently come across a publicly viewable project on Github that has no license associated with it. In this repo, there is a file with the logic and most of the code needed to work as a piece of a project I am working on. Not verbatim, but about 60% of it I'd like to use with various modifications. Once my code base is a little bit more stable, I plan to release what I've done under the WTFPL License. I've emailed the repo owner, and so far have not gotten a reply. I know I have the rights to fork the repo, but if I release a stripped down and modified version of the other project's file with mine, under the WTFPL, am I infringing on copyrights?

Per Github's Terms of Service, by submitted a project on Github and making it viewable to the public, you are allowing other users to see and fork your project. Doesn't say anything about modifying, distributing, or using the fork. And at what point of modification to the original does it become owned by me?