Fokus emphasizes anything you select by covering the rest of the page with a semi-transparent black overlay.
You can try it out here http://lab.hakim.se/fokus/ before installing.
The plugin comes with a few options that can be accessed from Chrome's extension listing page. This allows you to:
- Adjust overlay opacity
- Specify a modifier key shift/ctrl/alt/cmd which must be held down to active the overlay
- Adjust the padding around selection