Possible to darken underlying web page while playing text?

I would really like a feature to "dim" the background web page while jetzt is in the foreground. I suspect this is easy to do with DOM but don't know enough myself to submit a patch. Could this be a configurable option?

Start by locating the foreground playback UI and the extension's configurable options, then check how the underlying page is represented in the DOM. Confirm the desired behavior in Chrome: the page behind jetzt is dimmed while playback is active, and the effect can be configured.

Written by the indexing model from the issue text.