adwaita in 18.04 is actually broken to begin with. adwaita-icon-theme only contains the scalable icons, which do not have complete coverage (e.g. the entire icon preview in our kcm is not finding anything of the defaultishy icons). there is adwata-icon-theme-full which also contains pixmaps, but isn't actually installed. so, adwaita-icon-theme on its own is rubbish useless

I currently have two options in my mind

a) neon-adwaita package provides,breaks,replaces adwaita-icon-theme and adwaita-icon-theme-full. this keeps the theme out of the system by default assuming we use breeze by default. the user can however still install the theme explicitly if they choose to.
b) excessive postinst logic in some package which: copies index.theme and diverts it and adds Hidden=true (hides from ui). This is more work and doesn't actually shrink the ISO size so I like this less

adwaita in ubuntu also pulls in ubuntu-mono and humanity-icon-theme. by eliminating adwaita we'll also get rid of those.

there also needs to be an assertion about the available icon themes somewhere (iso build hook or openqa, build hook is probably better)

Option a) seems to be working really really well.

Basically we seed an empty override package.
If the user wants to, they can install the actual package manual which will drop our override.

A similar approach can be taken for T8633 so I am actually thinking about merging this into one neon-masks source. The intel driver override to force the kernel driver is also using a similar approach, so it could probably also get folded in.