Chevereto is distributed among several layers that allow better abstraction of the code. In the outermost 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 outermost layer because is the layer which drags fewer effects in the code.
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, light, 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 on this framework. You can use any framework you want to craft your own front-end (theme).