[unlimit]

Signature: hooksecurefunc([table,] "function", hookfunc) Arguments: table - A table object that contains the function to be hooked (table) function - The name of the function to be hooked (string) hookfunc - The function to be called each time the original function is called (function)

So every time, in your case, GridFrame:RegisterFrame is called, the function you specify will also run along side of it. (I think.)

Also, I think half of the advice I gave you is also wrong. <.< Setting it as a dependency is right, but honestly I don't really know where to put the hooksecurefunc now that I look at the code? Possibly just:

local function SkinGrid()
    local GridFrame = Grid and Grid:GetModule("GridFrame")
    if GridFrame and GridFrame.registeredFrames then
        
        hooksecurefunc(GridFrame, "RegisterFrame", function(frame)
            CreateBorderLight(frame, LeUI.media.bordersize, LeUI.bordercolor, LeUI.bordercolor, LeUI.bordercolor, 2)
            print("Adding borders to Grid frame")
        end)
 
        return true
    end
end
 
if not SkinGrid() then
    local f = CreateFrame("Frame")
    f:RegisterEvent("ADDON_LOADED")
    f:RegisterEvent("PLAYER_LOGIN")
    f:SetScript("OnEvent", function()
        if SkinGrid() then
            f:UnregisterAllEvents()
            f:SetScript("OnEvent", nil)
            SkinGrid = nil
        end
    end)
end

>.> I'm still learning here too.