Since the official Xbox 360 marketplace is retired, the community relies on these third-party sources: Aurora - ConsoleMods Wiki

An Aurora theme is a packaged file with a .xzp extension. This file contains all the visual and audio assets required to change the look and feel of the Aurora dashboard. Unlike official Xbox 360 themes, which only change the stock dashboard background and guide colours, Aurora themes can modify:

Aurora will prompt you to restart the dashboard to apply the changes. Agree to the restart. Troubleshooting Common Theme Issues