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? 
And at what point of modification to the original does source code with no license become owned by me?
licensing;open source;github
Copyright gives the author (or the copyright holder, depending on jurisdiction) pretty much exclusive rights. Nobody else is allowed to do anything.There are some restrictions, for example around fair use. You are allowed to make a parody of the project for satirical purposes (freedom of speech), for example, and you are allowed to quote tiny sections of it in a scientific article (freedom of education). If you are in the EU, you allowed to study it for interoperability purposes, iff you need it and iff the author refuses to give you the necessary information to build an interoperable product. (Note that charging a fee is not refusal!) But other than that, you aren't allowed to do anything. What you are allowed to do is spelled out in the license, and if there isn't one, then you can't do anything.The GitHub TOS may or may not be construed as an implicit license, but even if this holds up in court as an actual copyright license, it still only gives you the rights that are listed in it: to read and fork the repo. Nothing more.And at what point of modification to the original does it become owned by me?That's a question for the courts. Your safest bet is: as long as your lawyers don't get paid significantly more than the other guy's, probably never. (If you are in the US, the history of BSD may be of interest to you.)