Problem/Motivation
Claro is new admin theme built as part of the Admin UI and JavaScript modernization initiative. It is currently being developed in a Claro contrib project. As part of the project, we've worked on redesigning the toolbar: #3020422: Toolbar style update. While working on that issue we run into the issue that admin themes are unable to control the design of toolbar globally: #2539992: Move theme toolbar CSS to the Seven theme. It seems likely that the issue that would allow admin themes to control the toolbar globally isn't going to come up with a solution anytime soon, so the team started looking into alternative solutions.
Proposed resolution
Add alternative designs for toolbar in the Toolbar module that are enabled when Claro has been set as an admin theme. This requires shipping experimental code as part of a stable module. However, it will only be enabled when the contrib module (eventually experimental core theme) is enabled.
