You can use l0go's site at https://l0go.github.io/ghostty-base16-converter/ to generate the theme directly in your web browser, without installing anything.
Of if you want to generate the color config manually, first install Mustache.
Then run:
mustache base16.mustache
Then paste in a theme you copied from https://github.com/tinted-theming/base16-schemes into the stdin, then press CTRL-D.
Your theme will be printed to stdout.
Here is an automatic script that does this all if you're on Wayland on Linux:
wl-paste | mustache base16.mustache >> ~/.config/ghostty/config
Note that if you are on NixOS, I've created a flake that handles it all: https://github.com/RGBCube/ThemeNix.
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