Here is a Tampermonkey script I've used for some ime to make HN look the
way I want it to (plain white background).  It should be easy to adapt for
a dark preference.  It also adds a "bookmark" at the top of each page which
is also easily removed if you don't want that.

- - cut here - - - - cut here - - - - cut here - - - - cut here - - - -


// ==UserScript==
// @name         HN White
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Make HN background white
// @author       You
// @match        https://news.ycombinator.com/*
// @grant        none
// ==/UserScript==

function wide(s) {
    document.querySelectorAll(s).forEach(e => {e.style.width = "95%"});
}

function white(s) {
    document.querySelectorAll(s).forEach(e => {e.style.backgroundColor = "#FFFFFF"});
}

function rm(s) {
    document.querySelectorAll(s).forEach(e => e.parentNode.removeChild(e));
}

function bookmark() {
   var title_ = document.querySelectorAll(
       "#hnmain > tbody > tr > td > table.fatitem > tbody > tr > td.title > span.titleline > a"
   )[0].text
   var link_ = document.querySelectorAll(
       "#hnmain > tbody > tr > td > table.fatitem > tbody > tr > td.title > span.titleline > a"
   )[0].href
   var loc_ = document.location.href
   var d1 = (new Date()).toISOString().split('T')[0]
   var d2 = " /" + d1.substr(2, 2) + d1.substr(5, 2) + d1.substr(8, 2)
   document.querySelectorAll("body")[0].insertAdjacentHTML(
       "afterbegin","<pre>"+title_+" "+d2+"<br/>  "+link_+"<br/>  "+loc_+"</pre>"
   )
   document.querySelectorAll("head")[0].insertAdjacentHTML(
       "beforeend","<style>pre { max-width: none; }</style>"
   )
}

(function() {
    'use strict';
    wide("#hnmain");
    white("table");
    rm("#hnmain > tbody > tr:nth-child(1) > td > table > tbody > tr");
    rm("#hnmain > tbody > tr:nth-child(3) > td > table.fatitem > tbody > tr:nth-child(4) > td:nth-child(2) > form");
    bookmark();
})();