Both projects benefit. Tor gets funding to improve parts of the main Tor browser to meet Mullvad's specifications, Mullvad gets a new product and incidentally

Both projects benefit. Tor gets funding to improve parts of the main Tor browser to meet Mullvad's specifications, Mullvad gets a new product and incidentally gets to financially support a project that they obviously admire. The actual reskin is technically not all that challenging, for all that I'd still not want to do it just for fun. I'm sure that if Mullvad wanted to, they could have released a rebranded Tor Browser with their own development team. But that would have meant missing out on what is probably the main point of the exercise, which is giving more¹ money to the Tor project to make things better for everyone. [1]: See also https://www.torproject.org/about/membership/ https://www.torproject.org/about/membership/, wherein we discover that Mullvad gives Tor a minimum of $100k/year for membership.