If you try to open an already defined template within the Design View, it is possible that you will see only Custom HTML blocks. This is because the old templates were using the grid-system layout instead of the new one (d-flex).
The Design View supports only the d-flex layout, you are free to keep using the grid-system, but in case you want to switch to d-flex, here is an example you can follow to convert templates.
OLD TEMPLATE
<div class="row">
<div class="col-md-3 col-sm-3 col-xs-12">
<thing-details-widget>...</thing-details-widget>
</div>
<div class="col-md-3 col-sm-3 col-xs-12">
<thing-details-widget>...</thing-details-widget>
</div>
<div class="col-md-3 col-sm-3 col-xs-12">
<gauge-widget>...</gauge-widget>
</div>
</div>NEW TEMPLATE
<div class="d-flex flex-wrap components-row">
<thing-details-widget>...</thing-details-widget>
<thing-details-widget>...</thing-details-widget>
<gauge-widget>...</gauge-widget>
</div>As you can see, it is enough to remove all the intermediate DIV elements representing columns and change the root DIV to use the new d-flex layout.