Child Theme

A child theme is a theme that inherits the functionality and styling of Impreza. Child themes are the recommended way of modifying an existing theme. Read the WordPress Codex for details.

If you don't override PHP code, then it's better to add your custom CSS, HTML, JavaScript code via Theme Options for the entire site or for particular pages, these changes won't be overwritten during Impreza updates, read the relevant article.

There is a chance that your customization will stop working after an update so you should keep an eye on the changes that you applied. And always create full site backups before updating to be safe.