A Classic Theme is a presentation layer over WordPress-managed content. That distinction is more important than the shape of a toolbar.
Osbirds uses the Template Hierarchy instead of replacing it. It registers menus instead of hiding navigation in PHP. It registers Widget Areas instead of hard-coding every contextual panel. It uses the Customizer for bounded appearance choices and leaves editorial decisions in Posts, Pages, Categories, Comments, and Media.
The result is an interface that can be changed, audited, and rolled back without turning a theme update into a content migration.