HazeOver is a Mac app that dynamically dims all background windows. It feels a bit odd at first, but for most work it's a pleasant alternative to fullscreen apps. With just a bit of dimming I can still see my browser in the background or other Finder windows, but it's easy on the eyes.
HazeHover is simple but not without control. Hot keys and opacity sliders are nice options. I can turn off the effect with a hotkey or visit to my meny bar.
HazeOver is $4 on the App Store. I was provided a free promo code which I'm sure influenced my willingness to try the app. I also happen to think it's a nice addition to my Mac.