LUA:

local mod = Chatter:NewModule("Opacity Change")
local L = LibStub("AceLocale-3.0"):GetLocale("Chatter")
mod.modName = L["Opacity Change"]
mod.toggleLabel = L["Opacity Change"]

local defaults = {
	profile = { opacity = 1 }
}

local options = {
   opacity = {
      type = "range",
      name = L["Chat opacity"],
      desc = L["Changes the chatbox opacity"],
      min = 0,
      max = 1,
      step = 0.1,
      bigStep = 0.2,
		get = function() return mod.db.profile.opacity end,
		set = function(info, v)
			mod.db.profile.opacity = v
			mod:SetOpacity(nil,v)
		end
   }
}

function mod:OnIntialize()
	self.db = Chatter.db:RegisterNamespace("ChatOpacity", defaults)
	for i = 1, NUM_CHAT_WINDOWS do
		local cf = _G["ChatFrame" .. i]
   end
end

function mod:SetOpacity(cf, opac)
	if cf then		
      self:ChangeOpacity(cf,opac)
	else
		for i = 1, NUM_CHAT_WINDOWS do
			cf = _G["ChatFrame" .. i]
         self:ChangeOpacity(cf,opac)
		end
	end
end

function mod:ChangeOpacity(cf,opac)
   -- cf:SetAlpha(opac)
end

function mod:OnEnable()
   self:SetOpacity()
end

function mod:OnDisable()
   self:SetOpacity(nil,1)
end

function mod:GetOptions()
	return options
end

function mod:Info()
	return L["Changes the global opacity of the chatbox"]
end


ERROR OUTPUT:

1. Chatter-1.0\Modules\ChatOpacity.lua:19: attempt to index field 'db' (a nil value)
AceConfigDialog-3.0\AceConfigDialog-3.0.lua:241: in function <...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0.lua:190>
AceConfigDialog-3.0\AceConfigDialog-3.0.lua:999: in function <...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0.lua:924>
AceConfigDialog-3.0\AceConfigDialog-3.0.lua:1287: in function `FeedGroup'
AceConfigDialog-3.0\AceConfigDialog-3.0.lua:1208: in function <...nfig-3.0\AceConfigDialog-3.0\AceConfigDialog-3.0.lua:1187>
(tail call): ?:
<in C code>: ?
<string>:"safecall Dispatcher[3]":9: in function <[string "safecall Dispatcher[3]"]:5>
(tail call): ?:
AceGUI-3.0\AceGUI-3.0.lua:217: in function `Fire'
...s\Ace3\AceGUI-3.0\widgets\AceGUIWidget-TreeGroup.lua:396: in function `SetSelected':
...s\Ace3\AceGUI-3.0\widgets\AceGUIWidget-TreeGroup.lua:103: in function <...s\Ace3\AceGUI-3.0\widgets\AceGUIWidget-TreeGroup.lua:99>: