Bevy UI Theming Agent Skill

Bevy UI theming

When to use

Load this when many nodes should share colors or a widget style. Store the palette in one resource and copy it onto each node's color components.

ECS shape

• Per-node colors: BackgroundColor, BorderColor, BorderRadius, BoxShadow, BackgroundGradient.
• Editor widgets: bevy::feathers is a collection of styled widgets for editors and inspectors. Game HUDs use Node plus the color components below.
• Window chrome: WindowTheme on Window picks the OS window variant (Light or Dark). That is window chrome, not a UI skin.
• Systems: one system writes the color components from the palette resource. A second system, chained before it, swaps the palette.

A panel carries fill, border, radius, and one drop shadow. The palette is the only color source.

#[derive(Component)]
struct Panel;

#[derive(Resource, Clone)]
struct UiPalette {
    panel: Color,
    border: Color,
    shadow: Color,
}

fn dark_palette() -> UiPalette {
    UiPalette {
        panel: Color::srgb(0.12, 0.13, 0.16),
        border: Color::srgb(0.35, 0.38, 0.45),
        shadow: Color::srgba(0.0, 0.0, 0.0, 0.45),
    }
}

fn spawn_panel(mut commands: Commands) {
    let palette = dark_palette();
    commands.spawn((
        Panel,
        Node {
            width: Val::Px(280.0),
            height: Val::Px(160.0),
            border: UiRect::all(Val::Px(2.0)),
            border_radius: BorderRadius::all(CornerRadius::circular(Val::Px(8.0))),
            ..default()
        },
        BackgroundColor(palette.panel),
        BorderColor::all(palette.border),
        BoxShadow::new(
            palette.shadow,
            Val::Px(0.0),
            Val::Px(4.0),
            Val::Px(0.0),
            Val::Px(12.0),
        ),
    ));
    commands.insert_resource(palette);
}

A danger marker is a type variation: same panel shape, different fill. The two queries are disjoint, so one system may own every BackgroundColor write.

#[derive(Component)]
struct Panel;

#[derive(Component)]
struct Danger;

#[derive(Resource)]
struct UiPalette {
    panel: Color,
}

fn paint_panels(
    palette: Res<UiPalette>,
    mut panels: Query<&mut BackgroundColor, (With<Panel>, Without<Danger>)>,
    mut danger: Query<&mut BackgroundColor, (With<Panel>, With<Danger>)>,
) {
    let danger_fill = Color::srgb(0.55, 0.15, 0.15);
    for mut color in &mut panels {
        color.0 = palette.panel;
    }
    for mut color in &mut danger {
        color.0 = danger_fill;
    }
}

Swap the whole palette, then let paint_panels copy it. The chain keeps the writer after the swap.

#[derive(Resource, Clone, Copy, PartialEq, Eq, Default)]
enum ThemeKind {
    #[default]
    Dark,
    Light,
}

#[derive(Component)]
struct Panel;

#[derive(Component)]
struct Danger;

#[derive(Resource, Clone)]
struct UiPalette {
    panel: Color,
    border: Color,
    shadow: Color,
}

fn dark_palette() -> UiPalette {
    UiPalette {
        panel: Color::srgb(0.12, 0.13, 0.16),
        border: Color::srgb(0.35, 0.38, 0.45),
        shadow: Color::srgba(0.0, 0.0, 0.0, 0.45),
    }
}

fn light_palette() -> UiPalette {
    UiPalette {
        panel: Color::srgb(0.93, 0.94, 0.96),
        border: Color::srgb(0.55, 0.58, 0.64),
        shadow: Color::srgba(0.0, 0.0, 0.0, 0.18),
    }
}

fn paint_panels(
    palette: Res<UiPalette>,
    mut panels: Query<&mut BackgroundColor, (With<Panel>, Without<Danger>)>,
    mut danger: Query<&mut BackgroundColor, (With<Panel>, With<Danger>)>,
) {
    let danger_fill = Color::srgb(0.55, 0.15, 0.15);
    for mut color in &mut panels {
        color.0 = palette.panel;
    }
    for mut color in &mut danger {
        color.0 = danger_fill;
    }
}

fn toggle_theme(
    keys: Res<ButtonInput<KeyCode>>,
    mut kind: ResMut<ThemeKind>,
    mut palette: ResMut<UiPalette>,
) {
    if keys.just_pressed(KeyCode::F5) {
        *kind = match *kind {
            ThemeKind::Dark => ThemeKind::Light,
            ThemeKind::Light => ThemeKind::Dark,
        };
        *palette = match *kind {
            ThemeKind::Dark => dark_palette(),
            ThemeKind::Light => light_palette(),
        };
    }
}

pub struct ThemePlugin;

impl Plugin for ThemePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(dark_palette())
            .init_resource::<ThemeKind>()
            .add_systems(Update, (toggle_theme, paint_panels).chain());
    }
}

A header uses a linear gradient instead of a flat fill. Stops are automatic, so they space evenly from top to bottom.

fn spawn_header(mut commands: Commands) {
    let gradient = LinearGradient::to_bottom(vec![
        ColorStop::auto(Color::srgb(0.15, 0.25, 0.45)),
        ColorStop::auto(Color::srgb(0.08, 0.1, 0.16)),
    ]);
    commands.spawn((
        Node {
            width: Val::Percent(100.0),
            height: Val::Px(72.0),
            ..default()
        },
        BackgroundGradient(vec![Gradient::Linear(gradient)]),
    ));
}

WindowTheme is OS chrome. It does not recolor Node entities.

use bevy::window::WindowTheme;

fn spawn_window(mut commands: Commands) {
    commands.spawn(Window {
        title: "Game".into(),
        window_theme: Some(WindowTheme::Dark),
        ..default()
    });
}

Scope

• Theme colors, borders, radius, shadow, and gradients are components you set from your palette.
• bevy::feathers skins editor widgets. It does not replace the palette resource on a game HUD.

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 free-floating struct outside the World |

| 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 |

| Shared UI colors | UiPalette plus one paint system | A new color written in every spawn system |

| A header wash | BackgroundGradient with LinearGradient | A second BackgroundColor on the same node |

| Editor chrome | bevy::feathers widgets | Reusing feathers as the game HUD skin |

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.
• Do not treat WindowTheme as a HUD skin. WHY: it only selects OS window chrome (Light or Dark).
• 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

The library pin on bevy-master stays 0.20.

Official Documentation

• Assets - Official asset catalog.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.
• UI examples - Nodes, colors, and widget samples.

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.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.