Although we don't currently have a feature that allows direct adjustment of the opacity of backgrounds, there is a workaround. If you want to lighten or darken a background:

1. Place a rectangular sticker onto your page, stretch it to cover the background 
2. Use color fill to change it to the color you want (e.g. white or black)
3. Set the sticker transparency to adjust the degree of lightening or darkening.

I.e. if you want a 50% lighter background, just cover it with a white sticker and set the sticker to 50% transparency. The example below shows a photo with a 50% white sticker over the left third and a 50% black sticker over the right third. The center is uncovered.

We have some colored rectangle stickers that were created for this express purpose. Just open the stickers panel in the asset browser to the left of the workspace and type 'swatch' into the search field.

This same technique works for photos. Just stretch a swatch sticker to the same dimensions and position as a photo and set the color and opacity as needed.