La fonction de suggestion automatique permet d'affiner rapidement votre recherche en suggérant des correspondances possibles au fur et à mesure de la frappe.
At the moment it isn't possible to access the template in the design manager, as with other custom templates using drag and drop type emails.
You can access the source code of text modules in case that helps.
source code
If you're looking to have further control over CSS beyond the capabilities in the drag and drop editor it may be more beneficial to use the classic editor moving forward!