Rollout begins mid-April and should be complete by late April.
How this will affect your organization
When a Teams app allows customization, Global and Teams admins will be able to customize that app to match tenant branding.
What you need to do to prepare
To determine which apps are customizable, go to the Teams admin center. Expand Teams Apps; select Manage apps; then check the Customizable column of the apps list and sort by apps that are customizable.
You might want to notify any admins on your team about this new capability and update your training and documentation as appropriate.