carbon-components-svelte/palimpsest/src/components/ThemePicker.svelte

<script>
  export let inline = false;

  import { onMount, afterUpdate } from 'svelte';
  import { Select, SelectItem } from 'carbon-components-svelte';
  import { theme } from '../store';

  onMount(() => {
    let currentTheme = localStorage.getItem('theme');

    if (currentTheme) {
      theme.set(currentTheme);
    } else {
      if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
        theme.set('g90');
      }
    }
  });

  afterUpdate(() => {
    localStorage.setItem('theme', $theme);
    document.documentElement.setAttribute('carbon-theme', $theme);
  });
</script>

<Select {inline} labelText="Theme" bind:selected={$theme}>
  <SelectItem value="white" text="White" />
  <SelectItem value="g10" text="Gray 10" />
  <SelectItem value="g90" text="Gray 90" />
  <SelectItem value="g100" text="Gray 100" />
</Select>