Replacement for Isolator
Edit: Only just saw this comment by the OP. I will leave this answer up in case anyone finds it useful.



Try HazeOver. It does what Isolator did: Puts a dimmable layer over the screen except for the current window (or all windows of the current app).

It is a paid app, available on the Mac App Store. It has been updated for El Capitan and works for me without issues. The developer is responsive and has implemented a feature I personally requested.

You can activate HazeOver via the menu bar, a keyboard shortcut or AppleScript. I personally use a simple Alfred workflow to set the dim intensity.

Here is what it looks like (bottom left is HazeOver's preference window):