How do I theme the teaser?
You should already have a template suggestions available like : 

&lt;!-- FILE NAME SUGGESTIONS:
   * node--2--teaser.html.twig
   * node--2.html.twig
   x node--BUNDLE--full.html.twig
   * node--BUNDLE.html.twig
   * node--teaser.html.twig
   * node.html.twig
--&gt;


Make sure you have settings.yml set to twig.config: { debug: true ... and you will see the suggestions in the source.
You can set suggestions for any hook like:

function MODULE_theme_suggestions_node_alter(array &amp;$suggestions, array $variables, $hook) {
 $node = $variables['elements']['#node'];
 $type = $node-&gt;bundle();
 $view_mode = $variables['elements']['#view_mode'];

 $suggestions[] = 'node__'.$type;
 $suggestions[] = 'node__'.$type.'__'.$view_mode;
}