Just do it the same way one would add a background to the health/power bar.
like this:
Code:
self.Resting.bg = self.Resting:CreateTexture(nil, "BACKGROUND")
self.Resting.bg:SetPoint("TOPLEFT",self.Health,"TOPLEFT",-1,1)
self.Resting.bg:SetPoint("BOTTOMRIGHT",self.Health,"BOTTOMRIGHT",1,-1)
self.Resting.bg:SetVertexColor(0, 0, 0, 1)
The idea is creating a second square that's behind yours and make it 1pixel (see red offsets), larger.