Willam van Weelden may know the answer for sure but I am reasonably confident in saying Bootstrap was not used. It should be the case that if you can modify the layout.css and the layout pages in Project Setup that you will achieve what you want.
The CSS and the HTML is complex so you will need a high level of HTML and CSS knowledge if you want to make changes beyond those that can be achieved from the layout editor.
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@petergrainge
The layouts don't use bootstrap. The layouts don't use any kind of CSS framework. It is entirely self-contained and optimised for help.
You can create a layout that does use it, there's nothing blocking it. But as Peter mentioned it, will require a thorough understanding of HTML and CSS. In my experience, most people take a look at what it entails and then hire me to do it ;-)
For a seamless experience, I can think of a few options. You can create a bootstrap layout (or without bootstrap) to match the design. You can also generate an XML layout and let your developers inforporate it into their application. I don't think any HAT has an output based on bootstrap.
Kind regards,
Willam