chromium/chrome/browser/resources/ash/settings/device_page/customize_button_dropdown_item.html

<style include="md-select settings-shared input-device-settings-shared">
  :host([selected]) #container {
    background-color: var(--cros-highlight-color-focus);
  }

  #container {
    align-items: center;
    display: flex;
    height: 36px;
    padding: 0 16px;
  }

  #container:focus,
  #container:hover {
    outline: none;
    background-color: var(--cros-highlight-color-hover);
  }
</style>
<div id="container" hidden="[[option.hidden]]" tabindex="-1"
    class="option-item" on-click="onDropdownItemSelected_">
  [[option.name]]
</div>