Append this to news.css;
@media (prefers-color-scheme: dark) {
body { background-color:#000; }
#hnmain { background-color:#000; }
a:link { color:#eee; }
a:visited { color:#b2b2b2; }
.default, .title, .subtext, .yclinks, .comhead { color:#a2a2a2; }
.admin { color:#eee; }
.pagetop { color:#222; } /* not inverted */
.c00, .c00 a:link { color:#eee; }
.c5a, .c5a a:link, .c5a a:visited { color:#a5a5a5; }
.c73, .c73 a:link, .c73 a:visited { color:#8c8c8c; }
.c82, .c82 a:link, .c82 a:visited { color:#7d7d7d; }
.c88, .c88 a:link, .c88 a:visited { color:#777777; }
.c9c, .c9c a:link, .c9c a:visited { color:#636363; }
.cae, .cae a:link, .cae a:visited { color:#515151; }
.cbe, .cbe a:link, .cbe a:visited { color:#414141; }
.cce, .cce a:link, .cce a:visited { color:#313131; }
.cdd, .cdd a:link, .cdd a:visited { color:#222222; }
.pagetop a:link, .pagetop a:visited { color: #000; } /* not inverted */
.topsel a:link, .topsel a:visited { color:#000; }
.subtext a:link, .subtext a:visited { color:#7d7d7d; }
.comhead a:link, .subtext a:visited { color:#7d7d7d; }
.hnmore a:link, a:visited { color:#7d7d7d; }
textarea { background-color: #000; color: #eee; }
}
Inverted most colours (ff - 5a = a5).
#eee for text colour.
#000 for bg (body, #hnmain)
#hnmain's background is set with an HTML attribute, !important might be necessary for some browsers? Not sure, but works on my machine(tm).
Edit: s/html/body/, wasn't necessary to select on <html>. Split body and #hnmain selectors.