Featured images in lightbox

Dear developer,

First thanks for the awesome free theme!
But I have a little suggestion and I hope that you are willing to implement it in the coming update.
I am developing this website and I want the featured images to be opened in a lightbox. So I edited the code to display the thumbnails to the following

// Display Post Thumbnail on single posts
function dynamicnews_display_thumbnail_single() {

	// Get Theme Options from Database
	$theme_options = dynamicnews_theme_options();

	// Display Post Thumbnail if activated
	if ( isset($theme_options['post_thumbnails_single']) and $theme_options['post_thumbnails_single'] == true ) : ?>

		<a href="<?php echo (wp_get_attachment_image_src((get_post_thumbnail_id()),'thumbnail-size', true)[0]) ?>" rel="testtest">
			<?php the_post_thumbnail(); ?>
		</a> 

<?php
	endif;

}

And that is working fine if I add it in the main theme but when the theme has an update it will be overridden. So I tried to declare the function in my child theme but I can not override the function because it is not ‘pluggable’ like this:

if ( ! function_exists( 'theme_special_nav' ) ) {
see here for more info.

If you could make all the functions pluggable so it is possible to override the functions in a child theme I would be very grateful.

Thanks in advance!

Wilmar