add theme to selector

selector.php

<?php
function om_sample_add_lightbox_theme( $themes, $type ) {

	// Let's make sure we're adding to the lightbox themes. If not, return early.
	if ( 'lightbox' != $type ) {
		return $themes;
	}

	$themes['sample'] = array(
		'name' => __( 'Sample Theme', 'optin-monster-sample-themes' ),
		'image' => plugins_url( 'images/icon.jpg', __FILE__ ), // Feel free to add any image you like at this point
		'file' => __FILE__,
	);

	// As with all filters, make sure to return the data.
	return $themes;

}