As a Scrum product owner I need themes to structure the backlog for a better overview of the backlog.

The description of Mike Cohn clearifies, that Themes should be, whald Eipcs are at the moment: a grouping for User Stories (and Epics):
"Finally, “theme” is a collection of user stories. We could put a rubber band around that group of stories I wrote about monthly reporting and we'd call that a “theme.” Sometimes it's helpful to think about a group of stories so we have a term for that."
(http://www.mountaingoatsoftware.com/blog/stories-epics-and-themes)

Acceptance criteria:

• There is a new Issue Type: "Theme"
• Themes are used like Epics today (v6.1): They filter the backlog (User Stories and Epics) and are displayed in an extra column. There is no need that they are affected by quick filters.