A modified version of the Wikipedia - Dark, blue, simple Stylish theme

DarkBlueMediawiki.css

html {
    background: #000 !important;
}
/* main elements */

html > body,
table,
tbody,
tr,
td,
h1,
h2,
h3,
ul,
ol,
li,
dl,
dt,
dd,
p,
blockquote,
q,
cite,
fieldset,
form,
label,
iframe,
header,
main,
footer,
nav,
article,
section,
caption,
center,
abbr,
sub,
sup,
font,
span:not([class]),
code,
/* make it stick on other browsers */

#content tbody,
#content tr,
#content td,
/* most of the page */

.noprint,
.vectorTabs,
.vectorMenu,
.portal,
.portal > .body,
/* further reading */

.reflist,
.refbegin,
.catlinks,
.references,
/* edit links */

.editsection,
/* edit page */

.option,
/* "from Wikipedia" */

#siteSub,
/* rare finds */

.toccolours,
#template-documentation,
.iezoomfix,
/* listen to article */

.infobox,
/* image blocks */

.thumb,
/* internal wiki images */

.image.image > img[src^="//bits.wikimedia.org/"],
/* fix images inside notes */

#bodyContent .metadata img,
#bodyContent .infobox img,
#bodyContent .noprint img,
#bodyContent .plainlinks img,
.collapsible img,
/* search */

#searchButton,
.suggestions-results,
.suggestions-special,
/* lang */

.search,
/* blog.wikimedia.org */

#column-content,
#simpleSearch > input,
/* www-subdomain */

.divTop10item,
/* mobile subdomain */

body.mobile div,
/* JS created UI */

.ui-dialog div,
.ve-ui-mwCategoryItemButton,
/* syntax highlighter fix */

.mw-geshi > div > pre,
/* almost everything else; div[rel] created by JS */

div[id="content"] div:not([rel]):not([style]) {
    background: none !important;
}
div,
tt,
a,
span[class]:not(.legend-color) {
    background-color: transparent !important;
}
#mw-head,
.vectorTabs:hover {
    background-color: #181818 !important;
}
#content,
#simpleSearch,
/* side */

div[id="content"] table,
#content .infobox,
/* edit */

#content .options,
.ve-ui-menuWidget,
.ve-ui-toolbar-bar,
/* tooltips */

.referencetooltip > li,
#content .tooltip-repeat,
/* notifications */

#content .mw-notification,
/* popups */

.ui-dialog,
.ui-widget-content,
.ve-ui-popupWidget-body,
/* keyboard select */

.imeselector-menu,
/* mobile */

#mw-mf-header,
#mw-mf-drawer,
#mw-mf-content-left,
/* wikivoyage */

#content .topbanner .name,
/* misc */

.ve-init-mw-viewPageTarget-toolbar-editNotices {
    background: #222 !important;
}
#mw-panel#mw-panel .portal.portal > .body,
.portal.portal > .body,
.toc.toc,
#toc.toc.toc:hover,
#p-personal[id]:hover,
.suggestions,
.vectorMenu > .menu > ul,
option,
kbd,
.keyboard-key,
#footer,
#mw-panel,
/* JS created UI */

.ui-widget-overlay,
.uls-menu,
.uls-search,
.ve-ui-frame-body,
#content .ve-ui-toolGroup-tools,
/* search related */

.mw-search-formheader,
/* notes */

.metadata.plainlinks,
/* tips on hover */

.tipsy,
.tipsy-inner,
/* mobile sites search */

#results,
.mobile > #header,
/* spoken articles */

.mediaContainer .control-bar,
/* flagged articles */

#content .flaggedrevs_short,
.flaggedrevs_short_details,
/* coordinates on hover */

.nourlexpansion > span > div[style],
/* code */

#content .mw-geshi,
pre {
    background: #181818 !important;
}
/* needs testing */

div[style*="background"],
#content tr:nth-child(2n),
tr:nth-child(2n) {
    background: rgba(20, 20, 20, 0.2) !important;
}
#content th,
th,
/* wiktionary */

.NavHead {
    background: rgba(120, 120, 120, 0.1) !important;
}
/* VisualEditor overlay */

.ve-ui-dialog {
    background: rgba(20, 20, 20, 0.6) !important;
}
.ve-ui-iconedElement-icon[class*="ve-ui-icon-"]:not(#I):not(.ve-ui-icon-down) {
    background-color: #999 !important;
}
/* chemical structures */

#mw-content-text > p img,
#mw-content-text > dl img,
/* math formulas */

img.tex {
    background: #eee !important;
    padding: 2px;
    border-radius: 3px;
}
/* most images */

.image > img[src^="//upload.wikimedia.org/"]:hover,
#bodyContent .image > .thumbborder:hover,
/* structures, signatures */

#mw-content-text > table > tbody > tr > td > .image > img,
/* history */

.filehistory a img,
#file img,
/* music notes on Guitar */

#bodyContent .infobox .image > img:hover,
/* other images */

img.thumbimage,
.thumb .image > img {
    background: #eee !important;
    padding: 0;
}
/* home text logo */

.central-textlogo > img,
#wm-home-header > img,
a[href="/wiki/Wikisource"] > img[alt="Wikisource"] {
    background: #eee;
    padding: 5px;
    border-radius: 8px;
}
#p-logo,
#www-wikimedia-org img,
.central-featured-logo-inner > img[title="Wiktionary"] {
    background: #333 !important;
    border-radius: 8px;
}
#p-logo:hover,
#www-wikimedia-org img:hover {
    background: #49c !important;
}
/* 21x21 */

.articleFeedback-rating-label,
.articleFeedback-rating-clear,
.articleFeedback-switch {
    border-radius: 11px;
}
/* fr */

img[src$="/Bluebg_rounded.png"] {
    visibility: hidden;
}
/* cite tooltips */

.referencetooltip > li + li {
    opacity: 0 !important;
}
.referencetooltip > li,
#pagehistory > li.selected,
#simpleSearch,
a.button.active {
    border-color: inherit !important;
}
/* history */

#pagehistory > li {
    border-color: transparent !important;
}
/* ancestor trees */

tr,
td,
/* main */

#content,
#searchform,
/* other */

abbr {
    border-color: #aaa !important;
}
td[style*="border"]:empty {
    border-color: transparent !important;
}
/* Geometric_Shapes */

.wikitable,
th[style],
td[style] {
    border-color: #777 !important;
}
/* notes */

.metadata {
    border-left-color: #4be !important;
}
/* blue */

.ambox-notice {
    border-left-color: #27a !important;
}
/* orange */

.ambox-content {
    border-left-color: #a72 !important;
}
/* yellow */

.ambox-style {
    border-left-color: #772 !important;
}
/* mobile fix */

.mobile > #footer,
/* annoyances */

.navbox {
    box-shadow: none !important;
}
/* places on maps */

a.image ~ div:not([class]) > div {
    color: #111 !important;
}
#content .errorbox {
    background: #511 !important;
}
#content .successbox {
    background: #151 !important;
}
#content .warningbox,
#content .usermessage {
    background: #551 !important;
}
.suggestions-result-current,
.suggestions-result:hover,
#anoneditwarning,
#anoneditwarningtest {
    background: #047 !important;
}
.diff-addedline {
    border-color: #5f5 !important;
}
.diff-addedline .diffchange,
.mw-plusminus-pos {
    color: #7f7 !important;
}
.diff-deletedline {
    border-color: #f55 !important;
}
.diff-deletedline .diffchange,
.mw-plusminus-neg {
    color: #f77 !important;
}
* {
    color: inherit !important;
    text-shadow: none !important;
}
html,
.tocnumber,
/* www-subdomain */

.central-featured em,
.central-featured small {
    color: #ccc !important;
}
hr {
    height: 0 !important;
    border: 0 solid #555 !important;
    border-width: 1px 0 0 !important;
}
a {
    color: #4be !important;
}
a:hover {
    color: #7df !important;
}
a:visited {
    color: #98f !important;
}
a:visited:hover {
    color: #baf !important;
}
a.new,
.error,
.newpage {
    color: #f55 !important;
}
a.new:hover {
    color: #f77 !important;
}
::-moz-selection {
    background: #9cf !important;
    color: #000 !important;
}
::selection {
    background: #9cf !important;
    color: #000 !important;
}
input[type="submit"],
input[type="button"],
input[type="file"],
input[type="reset"],
button,
/*exp?*/
/*.buttons span,*/

a.button,
.ui-button,
.ve-ui-buttonWidget {
    color: #fff !important;
    border: 1px solid #555 !important;
    background: #111 !important;
    border-radius: 7px !important;
    box-shadow: none !important;
    text-shadow: none !important;
    background-image: linear-gradient(rgba(255, 255, 255, 0.2), rgba(125, 125, 125, 0.2), rgba(0, 0, 0, 0.3)) !important;
}
input[type="submit"]:hover,
input[type="button"]:hover,
input[type="file"]:hover,
input[type="reset"]:hover,
button:hover,
a.button:hover,
.ui-button:hover,
.ve-ui-buttonWidget:hover {
    border-color: #49c !important;
    background-color: #000 !important;
    color: #49c !important;
    box-shadow: 0 0 1em #4be !important;
}
button,
input,
select,
textarea {
    -moz-appearance: none !important;
}
input[type="radio"],
input[type="checkbox"] {
    color: #4be !important;
    background: #222 !important;
    border-color: #999 !important;
}
input,
select,
textarea,
input[type="text"] {
    background: #181818 !important;
    color: #fff !important;
    padding: 2px !important;
    border: 1px solid #555 !important;
    box-shadow: none !important;
}
input[type] {
    border-width: 1px !important;
    border-style: solid !important;
}
textarea:hover,
input:hover,
select:hover,
textarea:focus,
input:focus,
select:focus {
    border-color: #09f !important;
}
select > button,
#searchButton,
#simpleSearch > input {
    background-color: transparent !important;
    border: none !important;
    box-shadow: none !important;
}
html:root option:checked {
    background: #9cf !important;
    color: #000 !important;
}
#preftoc > .selected > a,
.portal > h5,
.portal > h3 {
    border: 1px solid !important;
    border-radius: 5px;
    background: #181818 !important;
}
/* opacity should stay at .99 for WebKit browsers */

.portal.portal > .body,
#toc.toc.toc:hover,
.vectorTabs[id]:hover,
#p-personal[id]:hover,
html > body > .suggestions[class] {
    opacity: 0.99 !important;
}
#content .tex {
    filter: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" version="1.1">\
<defs><filter id="A">\
  <feColorMatrix type="hueRotate" values="180"/>\
  <feComponentTransfer>\
    <feFuncR type="table" tableValues="1 0 0"/>\
    <feFuncG type="table" tableValues="1 0 0"/>\
    <feFuncB type="table" tableValues="1 0 0"/>\
  </feComponentTransfer>\
</filter></defs></svg>#A');
    -webkit-filter: invert(100%) hue-rotate(180deg) !important;
}

#p-logo, #www-wikimedia-org img, .central-featured-logo-inner > img[title="Wiktionary"] {
  margin-top: 10px;   
}

html {
  background-color: #181818 !important;
}

#content {
  border: none !important;   
}

#p-navigation h3 {
  display: none !important;   
}

.portal h3 {
  border: none !important;   
}

* {
  border-color: #444 !important;
  border-width: 1px !important;
}