To properly edit the ghost default theme (either Casper / Source) you need to first run a "Yarn install" command inside the theme's root directory, and then, when you make edits to a css file, you should run "Yarn dev" command inside theme's root directory so that you changes get applied to /assets/built/example.css.
However, due to some reason (probably gulpjs not being present and a few other things) yarn install and yarn dev commands fail, and since cloudron's security policy doesn't allow installing something inside the docker container of cloudron apps, i had to create a post here to ask for your kind help.
Additional note:
There are many ghost themes available which don't require these stuff, However, most important themes, including all of the free and official Ghost themes and many premium themes use Gulp to compile CSS files. so, it would be best if you could pre install that so that yarn dev command could work out of the box on cloudron ghost app.