The page editor
Using the page editor you will have full access to all of the files that make up your theme.
The page editor really is for advanced used only and only if you have prior experience with HTML.
Our themes are built using a template language called Twig. Whilst most of the theme consists of HTML/CSS/JS, Twig is used to generate dynamic content. If you're familiar with HTML then you shouldn't have any problem in understanding Twig - and all aspects of how Twig works is documented in our help guides on advanced customisation.
Read more about using the page editor and editing your files using Twig by clicking here.