When authoring a Template, you can write everything from scratch, using your own HTML and CSS. Within the CSS tab you can write any styling code you need and it will apply to the HTML elements you insert in the HTML tab.
If you compare it to the most recent versions of Chromium, there isn't much difference when it comes to general CSS. This means that you're free to use things like Flexbox, Custom properties or CSS Grids!
PDFMonkey supports SCSS
When it comes to writing your styling code, you can choose to write classic CSS or decide to use SCSS as it provides some capabilities that CSS does not at the moment like selector nesting or class generation.