How to theme all pages after a certain url
You can use THEME_NAME_theme_suggestions_page_alter to create your own template suggestions!

In your theme:

File: my_theme/my_theme.theme

/**
 * Implements hook_theme_suggestions_HOOK_alter.
 */
function my_theme_suggestions_page_alter(&amp;$suggestions, $variables) {
  $current_path = \Drupal::service('path.current')-&gt;getPath();
  $current_path = \Drupal::service('path.alias_manager')-&gt;getAliasByPath($current_path);

  if (strpos($current_path, 'this-page') !== false) {
    $suggestions[] = 'page__this_page';
  }
}


File: my_theme/templates/page--this-page.html.twig

// Output html and stuff


Obviously replace "my_theme" with the name of your custom theme.

Here is a nice tutorial that expands more on it.