The lightbox feature is by default turned on, and you can switch it on or off by going to Dashboard > Elementor > Settings > Style, and make sure the checkbox is checked.

1. Inside the editor, drag and drop the Image widget, and choose an image
2. Change the link to 'Media file' (This has to be done for the lightbox to work)
3. Go to the left hamburger icon, and click on a new setting called Global Settings
4. Now, click on the lightbox tab. You will now be able to change the  background color
5  Preview the lightbox changes by clicking on the image.

You can also switch the Lightbox feature on and off from the Global Settings page in addition to the Elementor dashboard.