The theme is only displayed at the Portfolio level due to the System Role Toggles. It isn't shown under Enterprise but at the Portfolio level. This is why it may seem like the theme backlog is not visible in the Enterprise layer.

If I go to Enterprise > Backlog, the theme backlog is not visible. However, Epic backlog is visible on the portfolio layer, Theme backlog as well.

The customer expects to be able to view the Theme Backlog at the Enterprise layer without selecting a Portfolio.