Bevy UI theming

When to use

Load this when many nodes should share colors or a widget style. There is no Godot Theme resource in Bevy.

ECS shape

• Per-node colors opened: BackgroundColor, BorderColor, BorderRadius, BoxShadow, BackgroundGradient.
• Feature: bevy_feathers is a collection of styled widgets for editors and inspectors. Check that module for widget names before writing them.
• Window chrome: WindowTheme is an enum on bevy::window for the window theme variant. That is OS window chrome, not a UI skin.
• Systems: you set components. No theme inheritance system was named.

Coming from Godot

Godot idea	Bevy idiom
Theme resource	Your own resource of colors, copied onto BackgroundColor. No Theme type was opened.
Style box	BorderRadius and BoxShadow on a Node.
Editor widgets	bevy_feathers exists. Do not assume it skins a game HUD.

Scope

• Theme, stylebox, and font-theme data are yours to build. Check bevy::ui before assuming a built-in type.
• Check the feathers docs for widget pages before writing them.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A Godot node script
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot Theme or StyleBox types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Godot types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

docs.rs links use the unversioned latest permalink. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Migration guides - Unversioned hop index.
• bevy crate - API reference and Cargo feature list.
• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.
• bevy::ui - Color and radius components.
• bevy::feathers - Editor widget collection.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-ui-layout - Nodes being styled.

Complements

• bevy-ui-text - Font choices.
• godot-ui-theming - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.