Closing all channels should do it. You can also unlist your snap in the settings tab on snapcraft.io. More info: Remove snap from store
If I understand you correctly, you want a user to install a system-wide theme by installing a snap? You cannot do this from the snap itself, but this should be possible by the user. We did something similar for the communitheme snap (the initial name of the Yaru theme). The end of this blog explains how it works: https://didrocks.fr/2018/04/10/welcome-to-the-ubuntu-bionic-age-new-wip-ubuntu-theme-as-a-snap/
Basically, you can add the share directory of your snap to XDG_DATA_DIRS and your theme will be available system-wide. If you want to check it out, this is still available on Ubuntu 18.04. Later releases shipped Yaru, so the session which set the XDG_DATA_DIRS was removed. I’m not sure if you can do it without a session file, because GNOME shell needs to pick up the XDG_DATA_DIRS variable and that won’t happen if you set it after GNOME shell starts.
If you’re interested, take a look at these files on Ubuntu 18.04
/snap/communitheme/current/session
/usr/share/wayland-sessions/ubuntu-communitheme-snap-wayland.desktop
/usr/share/xsessions/ubuntu-communitheme-snap.desktop
# + this stuff is added to in gsettings: https://launchpadlibrarian.net/362687782/ubuntu-settings_17.10.19_18.04.diff.gz
Edit: just saw you mentioned Qt. I have no idea if something like this is possible on KDE…