iconOverlay

ext_localconf.php

/**
 * register hook for icon overlay
 */
	$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_iconworks.php']['overrideIconOverlay'][]
		= 'KayStrobach\\Themes\\Hook\\IconUtilityHook';

ext_tables.php

/**
 * add themes overlay
 */
	array_push($GLOBALS['TBE_STYLES']['spriteIconApi']['spriteIconRecordOverlayPriorities'], 'themefound');
	$GLOBALS['TBE_STYLES']['spriteIconApi']['spriteIconRecordOverlayNames']['themefound'] = 'extensions-themes-overlay-theme';

/**
 * add sprites
 */
	\TYPO3\CMS\Backend\Sprite\SpriteManager::addSingleIcons(
		array(
			'switch-off' => ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Icons/power_orange.png',
			'switch-disable' => ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Icons/power_grey.png',
			'switch-on' => ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Icons/power_green.png',
			'overlay-theme' => ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Icons/overlay_theme.png',
		),
		$_EXTKEY
	);
	

IconUtility.php

<?php

namespace KayStrobach\Themes\Hook;

use KayStrobach\Themes\Utilities\CheckPageUtility;

/**
 * Class PageNotFoundHandlingHook
 *
 * @package KayStrobach\Themes\Hook
 */
class IconUtilityHook {

	/**
	 * adds the overlay icon to a page with a theme set
	 *
	 * @param $table
	 * @param $row
	 * @param $status
	 * @return void
	 */
	public function overrideIconOverlay($table, &$row, &$status) {
		if ($table === 'pages') {
			if (CheckPageUtility::hasTheme($row['uid']) > 0) {
				$status['themefound'] = TRUE;
			}
		}
	}

}