I'd like to add a lightbox effect to the Pop Up

I have PopUp installed but don't know what it does not show up...

I'd like to add a lightbox effect to the pop up that happens here:


As you can see from the screenshot, not a good visual without making the outer box opacity 80% with black.

I tried adding this:

<div id="cboxOverlay" style="display: block; opacity: 0.9; cursor: pointer;"></div>

above the rest:`
<div id='<?php echo $popover_messagebox; ?>' class='visiblebox' style='<?php echo $style; ?>'>
<a href='' id='closebox' title='Close this box'></a>
<div id='message' style='<?php echo $box; ?>'>
<?php echo do_shortcode($popover_content); ?>
<div class='clear'></div>
<?php if($popover_hideforever != 'yes') {
<div class='claimbutton hide'><a href='#' id='clearforever'><?php _e('Never see this message again.','popover'); ?></a></div>
<div class='clear'></div>

But when I close the box the black does not go away.

Please advice.

Thank you,