This is defined in this file:
anon-meta-packages/debian/control at master · Whonix/anon-meta-packages · GitHub
Likely due to technical limitations (dependencies). (Some Qubes package depends on xterm, not Whonix, I think.)
The following applies:
Also:
There’s indeed necessarily a degree of arbitrariness. There’s many flavors between minimalism and overload. Due to this and above it’s mostly the top contributors of Whonix who get to “vote”.
Advanced users can install CLI version of Whonix and then either package hardened-desktop-environment-essential-gui
among others from anon-meta-packages/debian/control at master · Whonix/anon-meta-packages · GitHub or go for Other Desktop Environments - Whonix.
Mostly default theme. No theme customization yet due to lack of resources (developers making the changes). See also Xfce theming - a few suggestions - it “only” needs source code contributions.