CRT overlay now is a CanvasLayer, that can be put on the viewport over all.

scenes/main.tscn

@@ -1,6 +1,7 @@
-[gd_scene load_steps=2 format=3 uid="uid://dsqm17w8hg428"]
+[gd_scene load_steps=3 format=3 uid="uid://dsqm17w8hg428"]
 
 [ext_resource type="Script" path="res://scripts/World.gd" id="1_131aq"]
+[ext_resource type="PackedScene" uid="uid://cnp8h65468543" path="res://scenes/overlays/crt_overlay.tscn" id="2_xf15f"]
 
 [node name="World" type="Node2D"]
 script = ExtResource("1_131aq")
@@ -10,4 +11,6 @@ one_shot = true
 
 [node name="AudioStreamPlayer" type="AudioStreamPlayer" parent="."]
 
+[node name="CRT_Overlay" parent="." instance=ExtResource("2_xf15f")]
+
 [connection signal="timeout" from="Timer" to="." method="_on_timeout"]