What StyleBox name does a Panel use in Godot 4?

Panel and PanelContainer both use the theme item named panel.

What replaced add_stylebox_override in Godot 4?

add_theme_stylebox_override(name, stylebox).

Why does changing one StyleBox change every panel?

The StyleBox resource is shared. Duplicate it, or enable Local to Scene, before modifying it per node.

Fix: Godot StyleBox Not Applying to a Panel or Button

Quick answer: Each Control looks up StyleBoxes by a specific name: Panel and PanelContainer use panel, Buttons use normal, hover, pressed, disabled and focus. Override with add_theme_stylebox_override("panel", style) (Godot 3's add_stylebox_override no longer exists), and remember that a local override beats the Theme.

You create a StyleBoxFlat, assign it, and the Panel looks exactly the same. StyleBoxes are resolved through a lookup chain, and a wrong name anywhere in that chain fails silently.

Use the theme item name the Control actually reads

Control	StyleBox names
Panel, PanelContainer	`panel`
Button	`normal`, `hover`, `pressed`, `disabled`, `focus`, `hover_pressed`
LineEdit, TextEdit	`normal`, `focus`, `read_only`
Label	`normal`
ProgressBar	`background`, `fill`

Setting normal on a Panel or panel on a Button does nothing. In the Inspector, expand Theme Overrides > Styles on the node to see the exact names it supports.

Use the Godot 4 API

var style := StyleBoxFlat.new()
style.bg_color = Color("1e2433")
style.set_corner_radius_all(8)
style.set_content_margin_all(12)
$Panel.add_theme_stylebox_override("panel", style)

add_stylebox_override() was the Godot 3 name. For Buttons, override each state you care about, or the default theme still draws hover and pressed.

Know the lookup order

A Control looks for a StyleBox in this order:

Its own Theme Overrides.
The theme property on itself, then on each parent Control up the tree (a CanvasLayer or Node2D breaks the chain).
The project theme (Project Settings > GUI > Theme > Custom).
The default engine theme.

So a Theme assigned on a parent is ignored by a child that has a local override, and a theme set on a Control does not reach Controls under a different CanvasLayer.

Theme type variations

If you created a variation such as DangerButton in your Theme, the node must opt in through Theme Type Variation (theme_type_variation = "DangerButton"), and the variation needs its base type set to Button in the Theme editor.

The style applies but looks invisible

draw_center is off, or bg_color has alpha 0.
self_modulate or modulate makes it transparent.
A StyleBoxEmpty is assigned further up the chain.
You expected a ColorRect to change — ColorRect does not use StyleBoxes at all.

Editing one panel changes all of them

StyleBoxes are resources, shared by reference. If several panels use the same .tres, changing one changes all. Call style.duplicate() before modifying it at runtime, or enable Local to Scene on the resource.