Port more code over to use strict

plugin/src/Theme.lua

@@ -1,4 +1,6 @@
-local Theme = {
+local strict = require(script.Parent.strict)
+
+return strict("Theme", {
 	ButtonFont = Enum.Font.GothamSemibold,
 	InputFont = Enum.Font.Code,
 	TitleFont = Enum.Font.GothamBold,
@@ -9,12 +11,4 @@ local Theme = {
 	PrimaryColor = Color3.fromRGB(64, 64, 64),
 	SecondaryColor = Color3.fromRGB(235, 235, 235),
 	LightTextColor = Color3.fromRGB(160, 160, 160),
-}
-
-setmetatable(Theme, {
-	__index = function(_, key)
-		error(("%s is not a valid member of Theme"):format(key), 2)
-	end
-})
-
-return Theme
+})