This is a Gradience color preset, which includes CSS for undershoots. It is a dark theme, with green accents and nightly nature shades. It's easy on the eyes, not too dark, so you can tell windows apart easily. Headerbar is dark blue with bright yellow text (the stars!).

I suggest using Tela-green-dark icon theme.

The file is a JSON, which has to be placed in the Gradience's user preset folder. As far as I know, it may be either of the following:
[pre]
~/.config/presets/user (if Gradience is installed from the distro repo)
~/.var/app/com.github.GradienceTeam.Gradience/config/presets/user/ (if it is installed via Flathub)
[/pre]

Note: Gradience is an app to easily change the aspect of libadwaita apps, which allows to change the color theme and also apply custom CSS. The changes also apply, as far as possible, on GTK3 and 4 apps if you use adw-gtk3 theme.