You will need to install a user script manager extension to install this script.
When in "Work Offline Mode", the webpage you are on will display an "OFFLINE" Alert.
► The page will be mostly greyed out but still have some visibility.
► If you want the page to be greyed out completely so that if you leave your desk, no one will be able to see any details of the page you are on... such as emails...
then change number of opacity:0.95 to 1.0
► If you want the page to still be easily readable while in 'Offline Mode' yet still maintain the notification of Offline, then change the number of opacity to .1 or .05
★Written by Jefferson Scher. I just requested it and have permission to post it.