Before you get to that screenshot, you have to get past the big, bold sentence that says "A human-friendly alternative to netstat for socket and port monitoring on Linux.".
Nobody is questioning what themes are available on gnome. Including a screenshot of the software running in a window that very much looks like macOS X is simply misleading.
If a Rust crate uses anything from the OS, and doesn't mention that OS, I wouldn't expect it to work on that OS, regardless if it's Windows, Linux or macOS. Just like graphical crates state what APIs they support, and if Metal is not mentioned for example, it is most likely not supported.