However, independently from your reasoning below.
(Which I think which might be invalid but that is a rather minor point.)
Just the default xfce logo is somewhat inappropriate. While XFCE is amazing, the main point of Whonix isn’t a fashion nice looking distribution focusing on that. However, that’s also not a very big importance.
If it’s sourced from packages.debian.org, it’s trusted anyhow.
Even if not.
That’s already violated by using arc-theme which ships lots of artwork.
And even without arc-theme, in theory any image / icon by any could include a backdoor already.