However if anyone decides to do this in Tor browser then I suggest that you also add some kind of soft pattern to the letterboxing space to make the separation from the page content clear at a glance. Right now it's hard to distinguish on pages that use white background, so on most pages.
The pattern could be even something Tor-related, such as little onion watermarks. This will also more clearly signal to users that this space around the page content is very much intentional and is part of Tor browser's mission to fight against fingerprinting. Right now there's a lot of users confused by the letterboxing, mistaking it for a bug.
When I tried doing this before, it seemed like it was going to be really difficult. But as we've refactored a bunch of XBL stuff I think it might have gotten easier.
If use the browser toolbox to select the with class 'browserStack' that has the window, and manipulate that element's bgcolor - that's what I would try to get working but I can't dig into this for the next several months.
This and other features relating to letterboxing should be made a bigger priority. Most people don't know what letterboxing is and many think it is a bug introduced in the new browser version. The current letterboxing affects the browsing experience more than you would expect and will make users search for the preference to turn off the feature completely which is not a good outcome.