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;