I would say it's a mistake, but you never know. This style has been used for github enterprise to make it "differentiate" from github.com, I don't think it's a good idea to make them look the same...
It's nowhere near the features and look of Insomnia but a pet project that I work on my spare time it's http://github.com/jsargiot/restman. The initial need was because Opera didn't had an extension of such kind.