Microsoft has moved the classic Advanced Settings menu to the new Power Platform Environment Settings app as part of the 2024 Power Platform Release Wave 1. The new app provides a modern, performant, accessible, secure, and extensible settings management experience. However, if you still need to access classic Themes to edit/update/publish/export or change a logo, you can do so by navigating to the Power Platform Admin Center, selecting your environment, clicking on Settings, and then clicking on Resources > All Legacy Settings. This will take you to the classic view where you can find Themes.
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