Lightroom Classic version 7.0.1 Windows 10 Creators edition (Spring)
In Library module / grid view pressing L key to go into lights out mode does not dim the grey broader around the thumb nail image. This is dimmed in Lightroom 2015
Evidently it's a bug. I have the same issue and posted this recently. Here's the link to the post with a little clarification - https://feedback.photoshop.com/photoshop_family/topics/is-there-a-way-to-display-your-thumbnails-in-...
Hopefully it gets fix soon. Peace!