The main stylesheet of the theme is called flatbook.css and located in the /assets/css/ folder. To catch up with the latest coding standards, FlatBook was written in Sass markup language. If you wish to modify the source .scss files and compile the main theme stylesheet again, feel free to watch the source files, located in the /assets/scss/ folder.

It is highly adviced to use the Online Autoprefixer tool on the css file, after you done working with it.