Try this...
Code:
local frame.border = CreateFrame("Frame", nil, FRAME_NAME)
frame.border:SetPoint("TOPLEFT", FRAME_NAME, "TOPLEFT", -1, 1)
frame.border:SetPoint("BOTTOMRIGHT", FRAME_NAME, "BOTTOMRIGHT", 1, -1)
frame.border:SetBackdrop({
bgFile = [[Interface\ChatFrame\ChatFrameBackground]],
edgeFile = [[Interface\ChatFrame\ChatFrameBackground]],
edgeSize = 1,
})
frame.border:SetFrameLevel(FRAME_NAME:GetFrameLevel())