This website is using slightly more of a front-end, sort of essential-yet-minimal. First of all there is official Tailwind CSS plugin Typography (requirement to be found in package.json file).
Custom module Ph* core is required, enclosed in the repository. Specifically it serves for a development of a set of very custom widgets (see in the previous section about these), as well as for definitions of some child PHP classes, in the best OOP manner with current Drupal. For fellow Drupalers it is known why/how theme and module are differently designed as code providers, some type of code is exclusively for modules, some configs and code organization are meant for themes etc.