First of all, although possible, I would suggest you do not make any changes to those files directly anyway. If you do that , you will have to apply your changes every time there is an update.
See here: https://docs.wp-pizza.com/advanced/?section=modify-css-styles-layout

Secondly, it sounds more like you have a permission issue of some sort. WPPizza does not handle anything that would allow you – or disallow you – to edit its files. This would be server side or some “security” plugin or some other wordpress constant perhaps that is blocking yo from saving
In other words, this has nothing to do with wppizza and I cannot help you there I’m afraid .