At the bottom of Photoshop’s Export As dialog (found in File > Export) there is an icon to the left of the zoom 
that has no tooltip when you mouse over it. Depending on the image you might not see any effect if you click it. It’s a pretty cryptic icon, so what does it do?
If your image has a transparent background, and you’ve chosen a format that supports transparency (such as SVG, PNG, or GIF), clicking that icon will toggle the background color between light and dark. It’s only for previewing, so it has no effect on the final exported image.
While I eventually figured out what it does, it would have been nice if Adobe would have added a tooltip to explain the button.