Enable dark theme (via firefox console - F12) in addon.mozilla.org aka AMO

script.js

// Navigate to any AMO webpage and run the script in console
// AMO/blog, AMO/etc.... are not supported.

function decimalToHex(decimal) {
  if (decimal < 16)
    return "0" + decimal.toString(16);
  else if (decimal > 255)
    return "00";
  else
    return decimal.toString(16);
}

var _elements = document.getElementsByTagName("*");

var _colorClassList = [
  "SecondaryHero-message-headline",
  "SecondaryHero-message-description",
  "SecondaryHero-module-description",
  "SecondaryHero-module-link",
  "Card-header-text",
  "SearchResult-link",
  "SearchResult",
  "Home-SubjectShelf-subheading",
  "Home-SubjectShelf-link",
  "LandingPage-addonType-name",
  "LandingPage-heading-content",
  "SearchContextCard-header",
  "Paginate-item",
	"Paginate-page-number",
  "AddonTitle",
  "PromotedBadge-label--recommended",
  "Addon-summary",
  "RatingManager-legend",
  "ReportAbuseButton-show-more",
  "Definition-dt",
  "Definition-dd",
  "DropdownMenuItem",
  "SectionLinks-dropdownlink",
  "SectionLinks-clientApp-android",
  "Card-contents",
  "LanguageTools-header",
  "Home-heroHeader-title",
  "Home-heroHeader-subtitle",
  "SearchResult-summary",
  "SearchResult--meta-section",
  "SearchResult-users-text",
  "ExpandableCard-ToggleLink",
  "SearchFilters-label",
  
  
  "MetadataCard-content",
  "MetadataCard-title",
  "AddonMeta-reviews-content-link",
  "AddonMeta-reviews-title-link",
  "AddonMeta-rating-title",
]
var _bgColorClassList = [
  "Page-content",
  "SecondaryHero-module",
  "Card-header",
  "Card-contents",
  "SearchResult",
  "LandingPage-header",
  "Card-footer",
  "Paginate",
  "AddonMeta-overallRating",
  "AddonsCard-list",
  "DropdownMenu-items",
  "LanguageTools-header-row",
  "LanguageTools-table-row",

]
var _bgColorEXClassList = [
  "SecondaryHero-module-icon",
  "SearchResult-wrapper",
  [
    "Paginate-item",
    "Button--disabled"
  ]
  
]
var _colorNodeList = ["A", "H1", "H2", "H3", "H4", "H5", "H6"];
var _bg_exception = ["Header-title"];

for (var i = 0; i < _elements.length; i++) {
  for (var k = 0; k < _colorClassList.length; k++)
    if (_elements[i].classList.contains(_colorClassList[k])) {
      var _colorStyle = window.getComputedStyle(_elements[i]).getPropertyValue('color');
      if (_colorStyle) {
        var _color = _colorStyle.slice(_colorStyle.indexOf("(") + 1, _colorStyle.indexOf(")")).split(", ");
        if (parseInt(_color[0]) < 128) {
          for (var j = 0; j < _color.length; j++) {
            _color[j] = 255 - _color[j] - 24;
          }
          _elements[i].style.color = "#" + decimalToHex(_color[0]) + decimalToHex(_color[1]) + decimalToHex(_color[2]);
        }
      }
    }
  var _backgroundStyle = window.getComputedStyle(_elements[i]).getPropertyValue('background');

  for (var k = 0; k < _colorClassList.length; k++)
    if (_elements[i].classList.contains(_bgColorClassList[k])) {
      if (_backgroundStyle) {
        var _color = _backgroundStyle.slice(_backgroundStyle.indexOf("(") + 1, _backgroundStyle.indexOf(")")).split(", ");
        if (parseInt(_color[0]) > 128) {
          for (var j = 0; j < _color.length; j++) {
            _color[j] = 255 - _color[j] + 24;
          }
          _elements[i].style.background = "#" + decimalToHex(_color[0]) + decimalToHex(_color[1]) + decimalToHex(_color[2]);
        }
      }
    }
  else if (_elements[i].classList.contains(_bgColorEXClassList[0])) {
    /* if (_elements[i].style.background != "")
      _elements[i].style.background = "";
    else */
    _elements[i].style.background = "#ffffff";
  } else if (_elements[i].classList.contains(_bgColorEXClassList[1])) {
    var _color = _backgroundStyle.slice(_backgroundStyle.indexOf("(") + 1, _backgroundStyle.indexOf(")")).split(", ");
    _elements[i].onmouseover = (function(target, color) {
      return function() {
        target.style.background = color;
      };
    })(_elements[i], /* _color[0] == "255" ? "#f9f9fa" : */ "#1e1e1d");
    _elements[i].onmouseout = (function(target, color) {
      return function() {
        target.style.background = color;
      };
    })(_elements[i], /* _color[0] == "255" ? "#ffffff" : */ "#181818");
  } else if (_elements[i].classList.contains(_bgColorEXClassList[2][0])) {
    var _color = _backgroundStyle.slice(_backgroundStyle.indexOf("(") + 1, _backgroundStyle.indexOf(")")).split(", ");
    _elements[i].onmouseover = (function(target, color) {
      return function() {
        target.style.background = color;
      };
    })(_elements[i], /* _color[0] == "255" ? "#cbc5bf" : */ "#4c8258");
    _elements[i].onmouseout = (function(target, color) {
      return function() {
        target.style.background = color;
      };
    })(_elements[i], /* _color[0] == "255" ? "#ffffff" : */ "");
  }
}