To make edits to your Ion AP3 Theme, you will need to use a child theme. This ensures that any edits you make will not be overwritten when you update.
You can add custom CSS to the style.css file, or custom functions to the functions.php.
Important: changes to your child theme do not affect your app toolbar, menu, or custom pages. It only affects the content area for WordPress pages. To make global CSS changes to your app, you should use the custom CSS box in the app customizer (under the colors tab).
To edit a template file, copy the file from the parent theme to the child theme and make your edits there. For example, there is a content-list.php file in the sample child theme you can edit. This will change the way lists are displayed on your WordPress pages.
To use the child theme, simply upload it to your wp-content/themes folder, or add it under Appearance => Themes => Add New => Upload. You do not need to activate it, the app will automatically detect and use your child theme as long as you don't change the folder name of ion-ap3-child.
Changes not taking effect?
Your child theme stylesheet tends to be cached in the app. If you make changes but are not seeing them, increase the version number in your child theme functions.php file and reload.
Please also note that your child theme only affects WordPress pages, you will not see any changes to custom pages, or your toolbar or menu.