This plugin displays your images in embedded pop-up windows when you click on them, darkening your website in the background, instead of taking you to another window.
Go to the Appearance > Lightbox Plus menu to access the settings. You can change the style of the Lightbox Plus pop-up in the first panel, Lightbox Plus – General Settings. We recommend changing the style to “Black,” but you may find other styles more to your liking.
To test a different style, simply change the setting, then open up one of your pages with image links (i.e. a photo gallery) in a new tab or window, and click on the image. Each time you change the style setting, you will need to refresh the page with the image link.
More information: https://wordpress.org/plugins/lightbox-plus/