How can I render a responsive image style from an image in the theme directory?
This is the same when using the image filters from the Twig Tweak
  module: You can't render responsive images from a random URI, it has
  to be an URI that is stored in the file_managed DB table. But with
  Twig tweak you can render normal image styles from random URIs, and
  manually create a responsive image tag by rendering multiple normal
  image styles. – Hudri


Great, so there is a workaround one can do, manually create one:

{% set imagePath = '/themes/custom/mytheme/img/mygraphic.png' %}
&lt;img srcset="{{ imagePath|image_style('thumbnail') }} 300w, {{ imagePath|image_style('medium') }} 800w" src="{{ file_url(imagePath) }}" alt="some graphic image" typeof="foaf:Image"/&gt;