Gtk3 App Python theme and font support

How to properly support for theme installed by the user?
How can I properly load fonts, themes, and icons from the system?

When running normally, everything works fine. But on installing from the snap, it looks weird.
How can I properly configure it? There are a couple of similar threads, but all are left unanswered.

Here is the https://github.com/cijo7/Halo/blob/master/snap/snapcraft.yaml.

you can test it by running:

$ sudo snap install --edge halo-weather

Try the Ark-Dark theme. It seems to have no effect.

Refer How to use the system GTK theme via the gtk-common-themes snap