There are no HTML files
Flarum is a Single Page Application rendered via javascript. You can alter the Mithril components (and the html they generate) via an extension.
But it you're not into that or only want to change the style, there is already a lot you can customize by using custom LESS (CSS) that you can input directly in Flarum's admin dashboard
If you decide to extend components via an extension, have a look at the existing extensions. Most of the stuff you might want to do can probably already be found in other extensions. Don't hesitate to ask if you have more specific questions ?
