http://ea5faa5po25cf7fb.onion/projects/tor/ticket/30812#comment:3

More details: turns letterboxing dark, check the profile location in about:profiles, cd there, mkdir chrome and paste the below into userChrome.css there.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* about:newtab and about:blank
 * also remove white flash: see https://bugzilla.mozilla.org/1488384 */
#browser vbox#appcontent tabbrowser,
#content,
#tabbrowser-tabpanels,
browser[type=content-primary],
browser[type=content] > html {
	background: #2A2A2E !important
}

Proof: https://onion.torproject.org/