CSS get removed when project init

grapesJS does not deal with css directly it first parses it into a CSSOM and uses that in the editor so a class will not show up in the code preview unless there is a component in the editor canvas using it. Your classes will be imported dynamically as they are used in the editor canvas.

You could load the stylesheet externally during init and remove it from the index.html altogether

canvas:{
   styles: [
      'path/to/stylesheet'
   ]
}

You can also check the SelectorManager and CssComposer api reference

Thanks @Ju99ernaut, do you know if there is anyway to add some css and make it available in the canvas and as well have it in the export?