Pages is carefully well thought UI frame work that is built on top of Bootstrap 4 and Angular 8+. Currently you are viewing the HTML and JQuery version with Bootstrap 4.
This part of the doc will help you to quickly start your project and will you a basic idea about how pages work. Pages HTML version comes with 5 Layouts and 9 themes.
Select the layout you want
{% page-ref page="introduction/layouts.md" %}
Select the theme you want
{% page-ref page="introduction/themes.md" %}