Incorrect (prop drilling):

// app/routes/dashboard.gjs
export default class DashboardRoute extends Route {
  model() {
    return { currentTheme: 'dark' };
  }

  <template>
    <Header @theme={{@model.currentTheme}} />
    <Sidebar @theme={{@model.currentTheme}} />
    <MainContent @theme={{@model.currentTheme}} />
  </template>
}