[lightsoff/gnome-3-30] snap: update grade and plus from ubuntu-desktop branch

[Jeremy Bicha <jbicha src gnome org>]

commit e2cc8c933de4b7814cf90f2652720e3415be19f6
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Wed Jan 30 18:23:22 2019 -0500

    snap: update grade and plus from ubuntu-desktop branch

 snap/snapcraft.yaml | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)
---
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 78e6d60..4bf4eea 100644
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -9,14 +9,22 @@ description: |
   Players who delight in turning off lights will enjoy the huge number of
   levels.
 
-grade: devel # must be 'stable' to release into candidate/stable channels
+grade: stable # 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:
+    interface: content
+    target: $SNAP/gnome-platform
+    default-provider: gnome-3-26-1604
+  gtk-3-themes:
+    interface: content
+    target: $SNAP/data-dir/themes
+    default-provider: gtk-common-themes
+  icon-themes:
+    interface: content
+    target: $SNAP/data-dir/icons
+    default-provider: gtk-common-themes
 
 slots:
   # for GtkApplication registration