[Miiru]

Originally Posted by lucro The frames won't be created until they're needed, so it may be sufficient to just hook the setup function AchievementAlertFrame_SetUp and adjust it there.

I got it working like this:

hooksecurefunc(AchievementAlertSystem,"ShowAlert",function()
            local achievementAlertPool = AchievementAlertSystem.alertFramePool
                for alertFrame in achievementAlertPool:EnumerateActive() do
                    alertFrame.Icon:Hide()
                    alertFrame.Background:ClearAllPoints()
                    alertFrame.Background:SetSize(512,64)
                    alertFrame.Background:SetTexCoord(0,1,1,0)
                    alertFrame.Background:SetParent(alertFrame) 
                    alertFrame.Background:SetPoint("Center",29,1)
                    alertFrame.Background:SetTexture("Interface\\Achievementframe\\miirgui_ach.tga")
 
                    alertFrame.Unlocked:SetTextColor(unpack(miirgui.Color))
                    alertFrame.Unlocked:SetFont(unpack(miirgui.small))
                    alertFrame.Unlocked:SetShadowColor(0,0,0,0)
 
                    alertFrame.Name:SetTextColor(1,1,1,1)
                    alertFrame.Name:SetFont(unpack(miirgui.medium))
                    alertFrame.Name:SetShadowColor(0,0,0,0)
                end 
        end)

I do not know if it's the best method, but it works just fine . I tried hooking the setup function, but the hook did not seem to fire.