Ubuntu – How to make the background dark while watching video

You'll have to patch the flash-player to send the appropriate signals, like Totem does. Unfortunately, most people have to use Adobe Flash, which is proprietary software.

But did you know that Totem supports watching videos from Youtube? It does deactivate the screensaver when you're playing stuff. You find it in the dropdown list that normally says playlist in the right-panel of Totem. Hope that helps. :)

Yes, 'idle' generally means there is no user input through the keyboard or mouse (or touchscreen).

There is also API available for applications to tell the system to inhibit idle screen blanking, for when in presentation, watching a movie, etc… However, not all applications make use of this API.