Cannot make changes to ionic.app.css after installing Sass?

I'm working on an app in ionic, and would like to customize the .CSS file that was generated by Sass after installing. When I open the .CSS file in my editor (Edge Code), make changes, and try to save the file I get:
...The permissions do not allow you to make modifications.
I imagine there is a command I can type into the Terminal to grant this permission, but now sure what. I'm pretty new to using CLI tools. Can someone please help?
Thanks!

Note: you should not edit the generated css files directly, since your changes will get overwritten.
Nevertheless, to fix the permission issue, you need to set your user account as the owner of the file: 
sudo chown yourUser www/css/ionic.app.css

Just replace yourUser with the short user name of your account.