Thanks for your appreciation, Vivek. :)
Hashnode is designed entirely in house, borrowing inspiration from Material design guidelines. You can read @fazlerocks answer here, shining light on Hashnode's initial design approach.
Currently @suleman our rockstar UX designer has taken charge of all expansions, and refinements of Hashnode's visual language.
Hope that answers your question. Feel free to let me know if you want to know anything more. :)