It’s CMD (command) + Click on a Mac.
(or)
to select all pixels. You can also use CTRL + click on the thumb, but the you have to use “Select Pixels” from the popup menu.

When all pixels are selected: THAN you NEED to add a NEW LAYER and fill the space inside the selected pixels with any color you like. When thats done, you do CMD + D to deselect the pixels and then you change the fill of the new layer to zero (0%) and the next step is to add a new WHITE stroke of 50% to the new layer.

@site-admin-owner,
Fix or update this tutorial please. You need to let people know they have to add a new layer. if not this tutorial doesn’t work out.