You have full control over the theme files and you can change them any way you want.
If you just want to add some simple CSS snippets to your website, you can easily use the Appearance → Customize → Additional CSS area. Everything will remain the same even if you update or change the theme.
Otherwise, if you want to make some adjustments and maintain them even if you update the product, then the best bet is to use a child theme. You can find further details about how to easily create on by having a look at this article.