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I think that arriving at a page like "Thank you, a confirmation email has been sent to you." should also show the client logo (there shoud be an option for uploading a logo under settings).
You can redirect users to a page of your choice instead of showing the default "Thank you, a confirmation email has been sent to you" page by setting Subscribe success page in list settings.
This way, you can style the page any way you want and include your client's logo if you wish.