Added small theme demo.

scenes/theme_gallery.tscn

@@ -0,0 +1,11 @@
+[gd_scene format=3 uid="uid://b3gfs3dear3an"]
+
+[ext_resource type="Script" uid="uid://cublhdarwsfbk" path="res://scripts/theme_gallery.gd" id="1"]
+
+[node name="ThemeGallery" type="MarginContainer" unique_id=1432651951]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("1")