The name for that is letterboxing. The Tor Browser (and the Mullvad browser, based on the Tor one, and Firefox as of v. 67 with an about:config flag) all support it.

There are also add-ons that perform the same basic function with some added customisability [0].

[0] https://addons.mozilla.org/en-US/firefox/addon/canvasblocker...