Make a part of your foreground element slightly transparent, and perhaps add a blur effect, to let the background show through.
I most often see this in operating systems. Windows 7 had transparent title bars on windows, and many panels in macOS apps, such as Finder, have see-through sections that let the desktop background through. I think the effect is used in these places more often because you'll be moving these windows around a lot, so you'll see the results of the effect more often. In a static website, for example, it's not often that elements move, but if they do, you could apply this effect to make things more visually interesting.
You might need to apply a blur to the area that is letting the background through, so that whatever background is there doesn't get in the way of reading foreground content.