chromium/chrome/browser/resources/side_panel/read_anything/menus/color_menu.html

<simple-action-menu
    id="menu"
    label="$i18n{themeTitle}"
    menu-items="[[options_]]"
    event-name="[[toolbarEventEnum_.THEME]]"
    current-selected-index="[[restoredThemeIndex_(settingsPrefs)]]"
    on-theme-change="onThemeChange_"
    initial-count="5">
</simple-action-menu>