How to change the theme of Snap Apps?

I am enjoying the new "snap" applications, but apps installed using this method doesn't seem to be using the system gtk theme I have set.

Some use Adwaita or worse, Raleigh.

How can I have them use the standard Ambience theme I'm using across the whole system?

The issue is being tracked here: themes currently don't work with snaps yet.

Snap now has GTK theming support, however it requires to have the same theme installed as a snap which you have chosen on your system.

The official gtk-common-themes snap contains Adwaita, Adwaita-Dark, Ambiance, Arc, Arc-Dark, Arc-Darker, Breeze, Breeze-Dark, Communitheme, elementary, HighContrast, Radiance, and Yaru.

There is also adapta-theme and adapta-gtk-snap as well as gtk2-common-themes for GTK2 apps.

If you're not using one of these themes, I believe you're currently out of luck.

I replaced the snap packages with the apt packages.

snap list

I do the following replacement:

sudo snap remove gnome-calculator gnome-characters gnome-system-monitor gnome-logs
sudo apt install gnome-calculator gnome-characters gnome-system-monitor gnome-logs

Now the theme defined in tweak applied to these apps.

In some cases, you can force the snap to start with a theme by using the enviromental variable like this

GTK_THEME=Adwaita-dark /snap/bin/libreoffice.writer