Normally the ambient (hollow bubble) icon is not themed, it is just the regular app icon put in a circle. If this is not what you’d like, there is a workaround.
Themed icons in ambient mode
Apply the themed bubble built into the Notification Icon plugin. This will force the same app icons to be used in ambient mode. The themed icon simply darkens the background of the icons in the active view:
- Settings → 1-click themes
- Select the Notification icons theme pack
- Select the theme (there is only 1)