Add index.html so this is registered as a block theme.

quadrat/block-templates/index.html

@@ -0,0 +1,5 @@
+<!-- wp:spacer {"height":10} -->
+<div style="height:10px" aria-hidden="true" class="wp-block-spacer"></div>
+<!-- /wp:spacer -->
+
+<!-- wp:template-part {"area":"header","className":"site-header"} /-->