Never, ever edit your exported theme! Seriously.
If you wish to make customizations to your theme, we strongly advise that you create a Child Theme in accordance with WordPress guidelines. Child Themes utilize the original theme – in this case, your exported CE3 Theme for WordPress – and allow you to safely implement changes, customizations and additions.
By using a child theme, you may protect your personalizations when upgrading to new versions of CE3 Theme for WordPress, allowing you to take advantage of new features and improvements without having to redo your own work. All customizations, whether large or small, belong in a child theme.
Note that CE3 Theme for WordPress does not store any styling in the style.css file, as do most WordPress themes; the file is only used to identify the theme to the WordPress dashboard (for theme switching). Instead, styling is in /resources/css/. Custom CSS is perhaps best applied via PHPlugins, as described here.
For more information, please consult the WordPress Codex entry on Child Themes.