Godot ProgressBar Color Change

extends Sprite2D

var health = 100.0

# Called when the node enters the scene tree for the first time.
func _ready():
    pass # Replace with function body.


# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
    position = get_global_mouse_position()
    health_update()

func health_update():
    health -= .1
    $ProgressBar.value = health
    if health < 20:
        $ProgressBar.get("theme_override_styles/fill").bg_color = Color.RED
    elif health < 50:
        $ProgressBar.get("theme_override_styles/fill").bg_color =Color.ORANGE
    elif health < 80:
        $ProgressBar.get("theme_override_styles/fill").bg_color = Color.YELLOW
    else:
        $ProgressBar.get("theme_override_styles/fill").bg_color = Color.GREEN