Chevereto is distributed among several layers that allows better abstraction of the code. In the most outer layer is the theme and in the most inner layer is the G\ Library framework. In a system like Chevereto you are free to edit almost anything but you should always start by the most outer layer because is the layer who drags less effects in the code editing.
Chevereto core is built on top of the G\ Library PHP framework which was developed by the creator of Chevereto. This foundation was made to allow Chevereto to be more robust, lightm agile and fast. You can find useful information about it in the G\ Library documentation. You should check that documentation if you are planning to extend Chevereto or use it as the base of your project.
Chevereto default theme is built on top of Peafowl framework which pretty similar to Bootstrap or any HTML/JS Framework. Peafowl was used for the default theme but Chevereto core has no dependency over this framework. You can use any framework you want to craft your own front-end (theme).