You can change the opacity for any individual layer from the Layers menu.
- Open the Layers menu from the toolbar.
- Tap the layer you wish to adjust. It will become highlighted.
- Tap the selected layer again to open the layer popup.
- Drag the opacity slider at the bottom of the popup to adjust the transparency.
Bonus: The layer popup also allows you to rename, merge, duplicate, lock, or select everything on the layer.