creating 3px border

Hey, I'm kind of starting with Lua and I am in the process of building a UI with Lua only addons. What I mean by that is no in game config.

I was wondering if somebody could help me achieve the look f.e. TukUi or qMinimap achieved. It looks like that


A 3px border with black at the outside and grey inside. I looked into the code of qMinimap but I mean nowhere near to understand it.
It would really help if someone could explain to me how to achieve this look/ or how qMinimap does it.

here some of the border code of qMinimap

Atleast I think it is.

local shadows = {
	edgeFile = "Interface\\Addons\\qMinimap\\media\\glowTex", 
	edgeSize = 4,
	insets = { left = 3, right = 3, top = 3, bottom = 3 }
}
function CreateShadow(f)
	if f.shadow then return end
	local shadow = CreateFrame("Frame", nil, f)
	shadow:SetFrameLevel(1)
	shadow:SetFrameStrata(f:GetFrameStrata())
	shadow:SetPoint("TOPLEFT", -4, 4)
	shadow:SetPoint("BOTTOMRIGHT", 4, -4)
	shadow:SetBackdrop(shadows)
	shadow:SetBackdropColor(0, 0, 0, 0)
	shadow:SetBackdropBorderColor(0, 0, 0, 1)
	f.shadow = shadow
	return shadow
end
function CreateInnerBorder(f)
	if f.iborder then return end
	f.iborder = CreateFrame("Frame", nil, f)
	f.iborder:SetPoint("TOPLEFT", 1, -1)
	f.iborder:SetPoint("BOTTOMRIGHT", -1, 1)
	f.iborder:SetFrameLevel(f:GetFrameLevel())
	f.iborder:SetBackdrop({
	  edgeFile = "Interface\\Buttons\\WHITE8x8", edgeSize = 1,
	  insets = { left = -1, right = -1, top = -1, bottom = -1}
	})
	f.iborder:SetBackdropBorderColor(0, 0, 0)
	return f.iborder
end
function frame1px(f)
	f:SetBackdrop({
		bgFile =  [=[Interface\ChatFrame\ChatFrameBackground]=],
        edgeFile = "Interface\\Buttons\\WHITE8x8", edgeSize = 1, 
		insets = {left = -1, right = -1, top = -1, bottom = -1} 
	})
	f:SetBackdropColor(.06,.06,.06,1)
	f:SetBackdropBorderColor(.29,.302,.29,1)
CreateInnerBorder(f)	
end

local function StripTextures(object, kill)
	for i=1, object:GetNumRegions() do
		local region = select(i, object:GetRegions())
		if region:GetObjectType() == "Texture" then
			if kill then
				region:Hide()
			else
				region:SetTexture(nil)
			end
		end
	end		
end