There's a huge variety of PADD designs seen on-screen. As far as I can tell, having a decal on the back is optional... In fact, there are many PADDs that are totally blank on the back and just painted a solid color.
Most TNG era PADDs were blank on the back but later seasons saw the additions of decals as well as pin striping details being added to the backs. Most DS9 versions had both details. Voyager PADDs were mostly blank on the back. Very few details were ever added over the course of the show.