Custom Javascript Since 1.19.0 BETA
You can integrate some javascript code that will be loaded for each user in Admin > Theme > Custom JS
Don’t imagine you can accomplish miracles because the templates are not designed to be easily modified, but don’t be afraid to open an issue or even better a PR to add some usage examples to this page.
Examples:
Integrate Plausible
var script = document.createElement('script');
script.defer = true;
script.src = "https://plausible.io/js/script.js";
script.dataset.domain = "yourdomain.com";
document.getElementsByTagName('head')[0].appendChild(script);