For those of you as annoyed with this change as I am, here's a script I cobbled together for greasemonkey, that puts things back to the original look.
Note that I'm no expert with javascript/CSS so this might be a really dumb way to do things, but it works for me.
// ==UserScript==
// @name SEblockquote
// @version 1
// @grant none
// @include https://japanese.stackexchange.com/*
// ==/UserScript==
function addGlobalStyle(css) {
var head, style;
head = document.getElementsByTagName('head')[0];
if (!head) { return; }
style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = css;
head.appendChild(style);
}
addGlobalStyle('blockquote { background-color: #fbf2d4 !important; }');
addGlobalStyle('blockquote::before { width: 0px !important; }');