Migrating from Ubuntu 18.04 to Ubuntu 20.04 (and core18 to core20): gtk theme lost?

Hello fellow snapcrafters.
I have a GTK app whose snap I was building under an Ubuntu 18.04 OS. I tried running this snap in a Ubuntu 20.04 OS and noticed that the gtk theme was lost this way (the app worked fine but looked much worse than in Ubuntu 18.04, like if it was using the gtk “no theme” appearance).

Then I migrated the snap generation to use Ubuntu 20.04 (and core20) instead of Ubuntu 18.04 (with core18), but this has not fixed the problem. Any hint on how to bring back the gtk theme to it?

The snapcraft.yaml file is here: https://github.com/nblockchain/geewallet/blob/frontend/snap/snapcraft.yaml

Is there a reason you’re not using the gtk-common-themes content snap?

You might get more information from relevant folks if you move this topic to the #snapcraft category