Introduction
Brainlab is currently based on Bootstrap Version 4.5.3 which was released in October 2020: https://blog.getbootstrap.com/2020/10/13/bootstrap-4-5-3/
This styleguide extends the official Bootstrap documentation with information that goes beyond default Bootstrap functionality.
You can find the official Bootstrap documentation here https://getbootstrap.com/docs/4.5/getting-started/introduction/
Everything that is custom tailored for Brainlab can be found in the left sidebar which is available by clicking on the "Toggle Menu" button on the lefthand side.