<main class="light">
  <header>
    <h1>Scope Demo 3: Child Elements</h1>

    <button>light-mode button</button>
  </header>
  <section class="dark">
    <h2>Nested "Dark Mode"</h2>
    <button>dark-mode button</button>
  </section>
</main>

.dark {
  --btn-color: plum;
  --btn-contrast: black;
  background: black;
  color: white;
}

.light {
  --btn-color: rebeccapurple;
  --btn-contrast: white;
  background: white;
  color: black;
}

button {
  background: var(--btn-color, rebeccapurple);
  color: var(--btn-contrast, white);
}




/* layout */
*, *::before, *::after {
  box-sizing: border-box;
}

html {
  font-family: avenir, roboto, helvetica, sans-serif;
}

main {
  font-size: calc(1em + 0.5vw);
  display: grid;
  min-height: 100vh;
  place-content: safe center;
  padding: 1em;
}

header,
section {
  padding: 1em;
}

section section {
  margin-top: 1em;
}

h1, h2, h3 {
  margin-top: 0;
}

button {
  border: none;
  border-radius: 0.25em;
  padding: 0.25em 1.5em;
  font: inherit;
  cursor: pointer;
}