The plugin includes some options to customize the visuals of the included lightbox.
Width and height of the Lightbox
You can find those options in the Settings page, under the Lightbox tab.
Background Color
To change the lightbox background color that overlays your page, you’ll need to use custom CSS. Add the following to the Custom CSS field of your map:
.goverlay { background:rgba(190, 111, 228, 0.46) !important; }
The example above will produce a transparent purple. You can change the color code to the one you want. Example below: