How we got a custom Drupal 8 theme up and running ...

In finder, navigate to this file by following the path structure shown and copy this file. Navigate back to your theme folder and inside the “templates” directory paste “page.html.twig”. Even though we have disabled the cache will will need to flush Drupal’s cache every time we create a custom template in our templates folder. This is where you have to make a decision. If you only ever want custom templates to live inside your theme then you will have to live with navigating back and forth between the “class” theme directory in “core” and your own theme directory every time you want to create a new template. You will also have to flush the cache each time you do so. Alternatively, you could copy the entire “classy” “templates” directory in to your own theme and flush the cache just once. The downside to this is that all of your templates will show as overridden and you will struggle to determine whether a template has been altered from its original state. Whatever, you decide to do, make a decision and stick with it - for better or worse. And once you have, go back to your site and do the usual “Configuration” → “Performance” → “Clear all caches” routine. Then navigate “Back to site”.