Yes, you can, but you'll need to clone or create a child theme. 

You can right-click on the theme in Design Tools you wish to edit to do so. 

If you're cloning the theme, it's an entirely new theme instance that's wholly disconnected from the original theme. 

On the other hand, a child theme inherits all of the files, modules, and templates from the original theme unless there is an instance that overwrites them within the child theme folder. 

If you purchase a theme from the HubSpot marketplace, we recommend creating a child theme instead of cloning it. By doing so, you'll still get all future theme updates.