Sometimes you want to remove the clutter. That's where Lightroom's Lights Out mode comes in. It blacks out or dims the Lightroom interface and everything else on the screen apart from the images in the main panel.
It's useful when you want to focus without distractions only on the image while you're selecting or editing. I also find it very handy as a quick way to do a basic slideshow for clients so that they're not getting distracted by Lightroom's interface or other thumbnails.
Using Lightroom's Lights Out Function
There are three modes: lights on, lights dim, and lights off.
It couldn't be any simpler to use. The simplest way I find it to just press L to cycle through the three modes. And it's such a simple shortcut to remember and doesn't require any keyboard gymnastics. You get something like this:
There is also, of course, a way to do it through menus: Window > Lights Out. And there's another keyboard shortcut you can use to toggle the dimming: SHIFT+CMD+L (Mac) or SHIFT+CTRL+L (Windows).
Customizing the Lights Out / Dim Level
There are a couple of aspects of the lights out and dimming functions that you can control. You can find these options under Preferences > Interface > Lights Out.
Firstly, you can set the fill color. By default, the lights out mode uses black. But you can change that to some extent, with a choice of black, white, dark gray, medium gray, or light gray. I personally like using the white setting, but you can choose whichever one you prefer.
You can also set the dim level, with options for 80 percent (the default), 90%, 70%, or 50%.
It works in every module: library, develop, book, slideshow, print, web, and even map. In the library module, it works with both the thumbnail grid view and the loupe view (single-image view). In the book module it shows the full page preview. In the web module it shows the preview web page. And in the print module it shows the preview canvas.
Something to be aware of is that it applies to the whole screen. So if you're using a large display where Lightroom is only filling part of the screen, the dimming function still dims everything else as well.