Add persistent light and dark themes

tests/ui_theme_test.gd

@@ -0,0 +1,40 @@
+extends SceneTree
+
+const Style := preload("res://scripts/ui/ui_style.gd")
+
+var failures := 0
+
+
+func _initialize() -> void:
+	Style.set_dark_mode(true)
+	var root := Control.new()
+	var background := ColorRect.new()
+	background.color = Style.BACKGROUND
+	root.add_child(background)
+	var panel := PanelContainer.new()
+	panel.add_theme_stylebox_override("panel", Style.panel_box(Color("172337"), Color("405475")))
+	root.add_child(panel)
+
+	Style.set_dark_mode(false)
+	Style.refresh_tree(root, true)
+	_assert(background.color.is_equal_approx(Style.BACKGROUND), "background should switch to light mode")
+	_assert(root.theme.get_color("default_color", "RichTextLabel").is_equal_approx(Style.TEXT), "rich text should use the light text color")
+	var light_box := panel.get_theme_stylebox("panel") as StyleBoxFlat
+	_assert(light_box.bg_color.is_equal_approx(Color("fffffe")), "custom panel surface should switch to light mode")
+
+	Style.set_dark_mode(true)
+	Style.refresh_tree(root, false)
+	_assert(background.color.is_equal_approx(Style.BACKGROUND), "background should switch back to dark mode")
+	_assert(root.theme.get_color("default_color", "RichTextLabel").is_equal_approx(Style.TEXT), "rich text should use the dark text color")
+	var dark_box := panel.get_theme_stylebox("panel") as StyleBoxFlat
+	_assert(dark_box.bg_color.is_equal_approx(Color("172337")), "custom panel surface should switch back to dark mode")
+
+	root.free()
+	quit(1 if failures > 0 else 0)
+
+
+func _assert(condition: bool, message: String) -> void:
+	if condition:
+		return
+	failures += 1
+	push_error(message)

tests/ui_theme_test.gd.uid

@@ -0,0 +1 @@
+uid://bm47u1leamc1h