This is only a temporary workaround until they hopefully revert the change. Just use Greasemonkey / Violentmonkey or any other custom CSS tool you may have.
Luckily, CSS offers a brightness filter that just seems made for issues like this one...
// ==UserScript==
// @name Save My Eyes
// @include https://*.stackexchange.com/*
// @include https://*.superuser.com/*
// @include https://*.serverfault.com/*
// @include https://*.stackapps.com/*
// @include https://*.mathoverflow.net/*
// @include https://*.askubuntu.com/*
// @grant GM_addStyle
// @run-at document-start
// ==/UserScript==
GM_addStyle ( `
.s-topbar .s-topbar--content .s-topbar--item > .svg-icon
{
filter: brightness(75%) !important;
}
` );
Using a brightness 75% setting seems quite acceptable: