What TUI library was used to create btop?

[Ashvith10]

Generally, TUI apps made with older libraries, have limited color palette and lacks gradient. I've seen a growing number of TUI libraries in Go and Rust that use full-color, and also support gradients. I wanted to learn about the library that was using in this app to add nice gradients and colors in C++ (and possibly C).

[imwints]

There is no library being used. There is this issue already