You can’t create tags that live outside a project because all tags (and data) must be contained inside a project. This is by design; it helps us keep the conceptual model simple, and make it easier to build features like project-level access control.
If you name your tags the same across projects, you can use search to aggregate results from multiple projects. The following search query will return highlights tagged with Positive in the Interviews project:
project:Interviews tag:Positive type:highlight
This query will return things tagged with Positive across all projects: