Chris McCoy

Dexter is a simple and minimal Lightbox for WordPress, It takes minimal effort to implement this into your site.

Easy Dexter Lightbox

Easy Dexter Lightbox Plugin for WordPress

Dabuuker

Show content in a modal after page load.
Usage:
Select ‘SN Popup’ from the admin menu, then Add New. Type a name for the popup and insert any content you like. If you’re done, click Publish.
Now find a page or post you’d like to see the popup on and edit it. There should be a new metabox called ‘SN Popup’, choose the settings you want and hit Update, then view your page.
Video: https://www.youtube.com/watch?v=t6lt6v3fuEQ
To change the styling of the modal, you can override its styles in your theme’s css. Or for more advanced styling you can dequeue the default css and replace it with your own.
For reference, the default unminified version of the css is located at /plugins/sn-popup/vendor/featherlight-1.3.4/src/featherlight.css
To dequeue the old css and enqueue your own, add something like this to your functions.php:
<pre><code>add_action('wp_enqueue_scripts','remove_featherlight_css'); function remove_featherlight_css(){ wp_dequeue_style('sn-featherlight'); wp_enqueue_style('featherlight-new-css','/path-to-new-css'); } </code></pre>

Easy Dexter Lightbox vs Soronet Popup