Im building a custom theme, with custom modules and was just about to start drafting the template for the first page.
I would like to use the Design Manager Drag and Drop UI for easy templating, but i noticed it is greyed out when I want to add a new template.
It all works fine when i create a HTML/HUBL template and code the drag and drop area's myself, but I would prefer to build the templates without writing the syntax for the area/section/column/rows etc. While the tooltip on the attached image is quite clear, I do see people adding them in their themes (e.g.: https://designers.hubspot.com/blog/introduction-to-the-design-manager )
You cannot use the drag and drop editor inside a theme. The first post you referenced was from 2018 and before HubSpot had the ability to create what we now call themes.