[lerb]

I found this line in "GridFrame.lua"

function GridFrame:RegisterFrame(frame)
	self:Debug("RegisterFrame", frame:GetName())

	self.registeredFrameCount = (self.registeredFrameCount or 0) + 1
	self.registeredFrames[frame:GetName()] = self:InitializeFrame(frame)
	self:UpdateFrameUnits()
end

Changing it to this did exactly what I wanted! Thanks for the hints Phanx

function GridFrame:RegisterFrame(frame)
	self:Debug("RegisterFrame", frame:GetName())
	
	CreateBorderLight(frame, LeUI.media.bordersize, LeUI.bordercolor, LeUI.bordercolor, LeUI.bordercolor, 2)

	self.registeredFrameCount = (self.registeredFrameCount or 0) + 1
	self.registeredFrames[frame:GetName()] = self:InitializeFrame(frame)
	self:UpdateFrameUnits()
end