Using the gtk-common-themes snap

snapcraft.yaml

name: gtk3-demo
version: '0.1'
summary: A demonstration application for features of GTK
description: |
  This snap contains the gtk3-demo application, using theme data
  provided by one or more external snaps.

grade: devel # must be 'stable' to release into candidate/stable channels
confinement: strict

plugs:
  gnome-3-26-1604:
    interface: content
    target: $SNAP/gnome-platform
    default-provider: gnome-3-26-1604:gnome-3-26-1604
  gtk-3-themes:
    interface: content
    target: $SNAP/share/themes
    default-provider: gtk-common-themes:gtk-3-themes
  icon-themes:
    interface: content
    target: $SNAP/share/icons
    default-provider: gtk-common-themes:icon-themes
  sound-themes:
    interface: content
    target: $SNAP/share/sounds
    default-provider: gtk-common-themes:sounds-themes

environment:
  XDG_DATA_DIRS: $SNAP/share:$XDG_DATA_DIRS

apps:
  gtk3-demo:
    command: bin/desktop-launch $SNAP/usr/bin/gtk3-demo
    plugs:
      - desktop
      - desktop-legacy
      - gsettings
      - wayland
      - x11

parts:
  app:
    after:
      - desktop-gnome-platform
    plugin: nil
    build-attributes:
      - no-system-libraries
    stage-packages:
      - gtk-3-examples
    override-build: |
      set -ex
      snapcraftctl build
      mkdir -p $SNAPCRAFT_PART_INSTALL/share/themes
      mkdir -p $SNAPCRAFT_PART_INSTALL/share/icons
      mkdir -p $SNAPCRAFT_PART_INSTALL/share/sounds
    stage:
      - usr/bin/gtk3-demo
      - share/icons
      - share/themes
      - share/sounds