chromium/ash/webui/personalization_app/resources/js/ambient/ambient_theme_list_element.html

<style include="common">
  :host {
    --theme-item-padding: 24px;
    --theme-item-width: 156px;
  }

  iron-list {
    /* Supports 3 thumbnails in one row. */
    width: calc(var(--theme-item-width)*3 + var(--theme-item-padding)*2);
  }

  @media(min-width: 720px) {
    iron-list {
      /* Supports up to 4 thumbnails in one row for landscape. */
      width: calc(var(--theme-item-width)*4 + var(--theme-item-padding)*3);
    }
  }
</style>

<h3 id="ambientThemeDescription" class="ambient-subpage-element-title">
  $i18n{ambientModeAnimationTitle}
</h3>
<iron-list items="[[ambientThemes]]" as="theme" grid id="grid"
    role="radiogroup" aria-describedby="ambientThemeDescription">
  <template>
    <ambient-theme-item tabindex$="[[tabIndex]]"
        role="radio"
        ambient-theme="[[theme]]"
        aria-checked$="[[getAriaChecked_(theme, selectedAmbientTheme)]]">
      </ambient-theme-item>
  </template>
</iron-list>