[Lombra]

Can be done in a number of ways.

If you don't need the background to be transparent a simple solution could be to create three textures; first one black, then one gray offset by 1 from each edge, and then another black offset from gray edge by 1.

Or for transparent background, you can create each edge individially. First a three pixel wide black texture, and then a gray 1 pixel wide one in the middle of each black one.

Or you can combine either of the above with backdrops, but you don't gain much by doing that.

Texture sub layers will come in handy.