Here:

/* ==UserStyle==
@name Dark mode for gog.com
@namespace github.com/openstyles/stylus
@version 0.9.2
@description Dark mode for gog.com
@author flayeddeath
@supportURL https://www.gog.com/forum/general/dark_mode_as_a_user_style
@license CC-BY-NC-SA-4.0 (https://creativecommons.org/licenses/by-nc-sa/4.0/)
==/UserStyle== */
@-moz-document url-prefix("https://www.gog.com/") {
.wrapper {
background: #222 !important;
}
.layout {
background: grey !important;
}
div.big_post_left,
div.big_post_right,
div.big_post_main,
div.content {
background: #222 !important;
}
span {
color: #aaa !important;
}
div.product-row__text,
div.product-tile__info,
div.filters,
div.tag-item,
ul.settings__nav {
background: #555 !important;
color: #aaa !important;
}
div.article__body,
li.archives-switch__link,
h1.article__title,
h2.article-section__title,
h3.archives-list__title {
color: #aaa !important;
}
time.article__date,
time.archives-list__pubdate {
color: #888 !important;
}
a.settings__nav-link.is-active {
background: #666 !important;
}
/* Submenu improvements by InkPanther @ gog forums */
div[class^="menu-submenu"],
div[class^="menu-submenu"]::before,
div[class^="menu-submenu"]::after,
div[class^="menu-header"] {
background: #444;
}
span[class^="menu-triangle"] {
border-bottom-color: #800080;
}
a[class^="menu-submenu"] {
color: #cc5;
}
a[class="menu-submenu-link"]:hover {
background: #000080 !important;
color: #32CD32 !important;
}
}

Spent about half an hour to make this. Work in progress, should work with any browser style add-on that is based on CSS (Stylish, Stylus). The top submenus don't change (at least with Stylus), can't fix that [Fixed by InkPanther]. Gradients fade to white and not transparent, haven't looked into that at all yet.

Licence: CC-BY-NC-SA 4.0.

This piece of shit forum doesn't seem to support bbcode properly, so you don't get a proper monospace code block.

Thanks. That works, good stuff.

Please add this attribute, it will be useful for those who'd want to open an issue or smth.
@supportURL https://www.gog.com/forum/general/dark_mode_as_a_user_style
It may be @homepageURL if you want.

Thanks, added. I was intent on posting this to userstyles.org, but that site seems to be practically dead, given the insane amount of spam and general shitposts in comments and an extremely old and insecure version of nginx they're running.

Thanks, please add license too:
@license CC-BY-SA-4.0 (https://creativecommons.org/licenses/by-sa/4.0/)