message SystemTheme

Which LavaUI Theme every client that uses system colours should wear.

The compositor does not invent palettes — dark, light and nebula live in LavaUI. This is only the name, so a settings app can change it and every Lava window hears, without the control plane carrying fifteen colour sliders. A client that paints its own paper (a terminal, a player) ignores the push, or remaps it in Theme.onSystemUpdate.