#6153641 Converting current theme bar display to new, more clear design

Changes proposed in this Pull Request

• This PR updates the “Current Theme” bar to the new design as described in #54075.

Testing instructions

• Navigate to /themes/{SITE_URL}?flags=theme/showcase-revamp
• Verify that the current theme banner displays as indicated above.
• Verify that previous (production) design does not display if feature flag is not enabled.

Related to #54075