In Startups Open Sourced, Tom Preston-Warner is interviewed and says to design every detail of every page of a project before touching any code. While graphic design and technical design are very intertwined, I think in Github's case the graphic design came first, or was at least equally weighted. I don't know how SourceForge came to be, but I wouldn't be surprised to find out the backend was designed before the front end design.
IIRC, the general wisdom during SourceForge's heyday was to design the backend before the frontend. I recall reading a zen programmer story about someone writing a finance package, who was mocked by the "wise" programmer for designing and writing the UI first, before having any finance-related code.
This is true to a fault: for a while many people asked to be able to change the code font, or at least for it to respect browser defaults. They resisted because that went against what the designer thought was best - which I think is a case of form trumping function.
