So you are saying snap will installed the required theme, by checking the system theme? Nice
but what about qt, to get gnome and qt apps to look same, QGnomePlatform is use, though there isn’t much theme that support QGnomePlatoform but will you support it?
for now there are only one theme, adwaita-qt.
Edited: What if we mount the theme folder (/usr/share/themes and /home/$user/.themes) and For Icons (/usr/share/icons and /home/$USER/.icons)? It can save the one from downloading multiple copies of theme in future.
qt to use gnome theme is done by this guy, adwaita-qt and qgnomeplatform