It depends on what you’d like to change. If you want to remove or restyle content, it’s likely you can do it with some additional CSS. If you’d like to add information or change the layout dramatically, then you’d need to edit template files.
In the Minima theme, I did edit the template for the topic list to reposition/remove some avatars. You can see that here: minima/desktop/header.html at main · discourse/minima · GitHub
If you’d like to use the Minima theme and add additional customizations, I’d recommend installing Minima and adding your customizations to it by creating a new theme component. This way you can still get updates to Minima without worrying about your changes being overridden.