Block definitions should be at the top level, not the included level

templates/forms/inputs/user-theme.html.twig

@@ -1,4 +1,3 @@
-{% block input_user_theme %}
 {% if 'theme' not in form.fields.hidden %}
     {% if col_width %}<div class="{{col_width}}">{% endif %}
         <div class="form-group">
@@ -17,5 +16,4 @@
             </div>
         </div>
     {% if col_width %}</div>{% endif %}
-{% endif %}
-{% endblock %}
+{% endif %}