Template override not working

I can not get template override to word. I followed the normal instruktions from drupal doc.
"Locate the template you wish to override., Copy the template file from its base location into your theme folder., (optionally) Rename the template according to the naming conventions in order to target a more specific subset of areas where the template is used., Modify the template to your likin"

copied themes/bootstrap/templates/block/block--system.html.twig to default/thems/bootstrap_sub/templates/block/block--system.html.twig, cleared cache, but block--system.html.twig is still loaded from themes/bootstrap/templates/block/

What am i missing?