LSJ:DarkTheme
Browser or OS selection
If you have selected dark mode on your operating system or browser, then you will automatically see this site in dark mode if you have on your desktop at least these versions: Chrome 76, Firefox 67, Safari 12.1; and for mobile at least these versions: Android Webview 76, Chrome for Android 76, Firefox for Android 67, Safari on iOS 13. See also here for Edge browser.
Using custom css via a browser extension
Dark theme 2 as discovered by Stylus extension. Once, discovered, clicking on the screenshot will install it
You can drop the following code in Stylus (an extension for Chrome, Firefox or Opera and you can also install it, as well as any other Chrome extension, on Edge browser) or any other styling extension (or access it directly from here via an extension), and have lsj.gr displayed in a dark theme. It is also discoverable via Stylus and other extensions when you visit lsj.gr, click on the extension icon and then click Find styles and when the style is found, clicking on it will install it (mind you, the userstyles.org site is often unresponsive with 504 Gateway Time-out errors, so you may not get results there). This code works for wikipedia sites as well. You can simply add the extra URLs it should apply to via the extension settings (just use wikipedia.org in the Applies to URLs on the domain option in Stylus). Dark theme 2 is specifically written for Timeless Mediawiki theme, so you get good results if you have changed your Wikipedia user settings to Timeless.
As you can see, there are 2 dark theme styles to choose from. Dark theme 1 is simpler and Dark theme 2 more elaborate.
Dark theme 1
*{background:#262626!important;border-color:#3e3e3e!important;color:#e9e6e6!important}a:link{color:#a7a15e!important}a:visited,pre{color:#a67b07!important}a:hover,h1,h1 *,h2,h2 *,h3,h3 *,h4,h4 *,h5,h5 *,h6,h6 *{color:#556b2f!important}b,b *,strong,strong *{color:#af5f00!important;font-weight:400!important}i,i *,li{color:#e9e6e6!important}button,input,select,textarea{background:#262626!important;border:1px solid #3e3e3e!important}ul{list-style-image:none!important}#simpleSearch label{background:0 0!important}.new a,a.new{color:#98bfe7!important}#mw-editbutton-link,span.oo-ui-iconElement-icon.oo-ui-icon-link{background-color:#ce5149!important}.mw-wiki-logo{background-image:url(/images/a/ae/LogoD.png)!important;background-repeat:no-repeat!important;display:block!important;content:''!important;width:11em!important;height:11em!important;background-position:50% 50%!important;margin:.75em auto 0!important}#trrandomquote::before{color:#796c22!important}.searchmatch{background-color:#e7eebd8f!important;color:#000!important}
Dark theme 2
html{background:#000!important;height:100%;font-size:1.1em;filter:brightness(100%) contrast(100%) grayscale(0)!important;-webkit-filter:brightness(100%) contrast(100%) grayscale(0)!important;overflow-x:hidden}body{margin:0;background:#000;color:#ccf}a{color:#c60}a:hover,a:visited{color:#c30}h1{color:#fff;text-shadow:1px 1px 1px #000;font-size:40px!important}h1,h2,h3,h4{color:#69f}#personal .dropdown b{font-weight:400}b,strong{text-shadow:1px 1px 1px #000}#f-list{text-align:center;font-size:14px}div.editOptions{border:#222;background:#555}#mw-header-nav-hack{border-top:solid 2px #333;background:#333}#mw-site-navigation .sidebar-inner{background:#333;border:0}@media screen and (min-width:851px){#p-logo-text a{position:fixed;right:75vw}}@media screen and (max-width:850px){#p-logo-text{margin-top:0}}@media screen and (max-width:1099px) and (min-width:851px){#p-logo-text a.long{padding-left:7px;float:left;font-size:23px!important}}@media screen and (max-width:1099px) and (min-width:851px){#mw-related-navigation .sidebar-inner{background:#222!important;border:solid #111}}@media screen and (max-width:1099px) and (min-width:851px){#mw-site-navigation .sidebar-inner{background:#222;border:solid #111}}#mw-site-navigation .sidebar-chunk{border:outset #222;background:#333;box-shadow:2px 2px 1px rgba(0,0,0,.5);border-radius:3px}@media screen and (max-width:851px){#mw-site-navigation .sidebar-chunk{padding:0 15px 5px 20px}}@media screen and (max-width:1099px) and (min-width:851px){#mw-site-navigation .sidebar-inner{background:#222!important;border:solid #111!important}}.mw-parser-output a.external{color:#f90}#searchInput{padding:.2em 4.5em .2em 2em!important;margin:0}#searchInput-container{background:#333;height:35px}#simpleSearch{border:inset 1px #000;border-radius:3px;background:#333;height:35px;width:48vw;position:absolute;left:0;right:0;margin-left:auto;margin-right:auto;top:7px}#user-tools{position:fixed;right:86px}@media screen and (min-width:851px){#user-tools{left:74vw;top:5px}}.mw-notification{background:#222;border:#111;background:#111;border-radius:5px;color:#999;width:200px}.postedit-container{position:absolute;left:0;right:0;margin-left:auto;margin-right:auto;padding:0;padding:15px 0 15px 15px;text-align:right}.postedit-icon{margin:auto;padding:0}.mw-wiki-logo{background-size:contain}#mw-header-container{background:#555}#mw-header{width:100%;max-width:100%;padding:0;position:static}@media screen and (max-width:850px){#menus-cover{background:#000}}@media screen and (max-width:850px){#personal h2{display:none}}#mw-content-container{background:#1d1d16}@media screen and (max-width:850px){#mw-content-block{background:#000}}@media screen and (max-width:850px){#site-tools h2{right:3em;top:.9em;padding-bottom:5px}}@media screen and (max-width:850px){#p-logo-text a.long{width:100%;top:7px;text-align:center;font-size:40px!important}}@media screen and (min-width:851px){#p-logo-text{width:29em}}@media screen and (min-width:851px){#personal h2:after{width:36px}}@media screen and (max-width:850px){#p-logo-text{position:static;left:0!important;right:0;margin-left:0;margin-right:0;width:100%;top:3.3em}}@media only screen and (min-width:992px) and (max-width:768px){#p-logo-text{right:74vw}}@media only screen and (min-width:900px) and (max-width:100px){#p-logo-text{right:74vw;top:3em}}@media screen and (max-width:900px) and (min-width:100px){#p-logo-text a.long{font-size:43px!important}}@media screen and (max-width:1700px) and (min-width:800px){#p-logo-text{left:12px!important}}@media screen and (max-width:1700px) and (mix:851px){#mw-site-navigation{background:#000}}#mw-content{background:#333;border:solid #946c20;-webkit-border-bottom-right-radius:4px;-webkit-border-bottom-left-radius:4px;-moz-border-radius-bottomright:4px;-moz-border-radius-bottomleft:4px;border-bottom-right-radius:4px;border-bottom-left-radius:4px}#mw-page-header-links li.selected a{color:#cc0}#mw-related-navigation .sidebar-chunk{background:#333;border:outset #222;box-shadow:2px 2px 1px rgba(0,0,0,.5);border-radius:3px}@media screen and (max-width:1099px) and (min-width:851px){#mw-related-navigation .sidebar-chunk{background:#333;border:outset #222;box-shadow:2px 2px 1px rgba(0,0,0,.5);border-radius:3px;padding:0 15px 5px 20px}}.color-middle-container,.ts-inner{padding:0}#p-logo-text{left:120px}#p-logo-text a.long{padding-top:5px;padding-left:6px;font-size:30px;text-shadow:1px 1px 1px #222}#p-search{width:70em;padding-right:33px}#personal .dropdown h3{line-height:25px}#personal h2{text-shadow:2px 2px 0 #333;padding-top:7px;font-size:24px;text-transform:lowercase}#personal .dropdown{background:#222!important;border:solid #000}.codeEditor-status{color:#000}.oo-ui-labelElement-label{text-shadow:1px 2px #333}#iframeframe{height:700px}#wpSave{text-shadow:2px 2px 2px #000!important}#mw-footer-container{background:#000;padding-top:1em}fieldset{background:#222!important;border:solid #111}input{background:#333;border:solid 1px #111!important;border-radius:2px;padding-left:5px;color:#999}.mw-ui-button.mw-ui-progressive,.mw-ui-button.mw-ui-progressive.mw-ui-checked,.mw-ui-button.mw-ui-progressive:active,button,input[type=button],input[type=submit]{border:solid 1px #333!important;border-bottom:0!important;text-shadow:0 0 0 #000!important;border-bottom:solid 1px #333!important}.oo-ui-flaggedElement-destructive{background:#111;border:1px solid #444;color:#f33!important;padding-left:15px!important;padding-right:15px!important}.oo-ui-flaggedElement-destructive:hover{background:#222;color:#c00!important}#searchInput{border:0!important}@media screen and (max-width:850px){#searchInput{padding:0 0 0 6px!important}}.oo-ui-textInputWidget input,.oo-ui-textInputWidget textarea{background-color:#333;color:#fff}.oo-ui-textInputWidget>.oo-ui-labelElement-label{line-height:.5em}#pagehistory li.selected{background-color:#555}.warningbox{background:#1d1d16}.new a:hover,.new a:visited,a.new:hover,a.new:visited{color:#f33}.mw-plusminus-neg{color:red}#mw-searchButton,#searchButton{border:0!important;left:38vw;right:0;margin-left:auto;margin-right:auto}#mw-footer-container a:hover{color:#ff3}#mw-footer-container a:visited{color:#ff6}#mw-footer .footer-places li{float:right;display:block}#mw-footer li,#mw-footer ul{margin:0 auto}#mw-footer #lastmod{float:left;left:25px}.diff-addedline .diffchange{background:#1d1d16;padding-left:7px}.oo-ui-buttonElement.oo-ui-labelElement>.oo-ui-buttonElement-button>.oo-ui-labelElement-label,.oo-ui-buttonElement.oo-ui-labelElement>input.oo-ui-buttonElement-button{text-shadow:0 0 0 #000!important}.oo-ui-labelElement-label{text-shadow:0 0 0 #000}.oo-ui-panelLayout-expanded{background:#222}.oo-ui-messageDialog-message{color:#999}.oo-ui-buttonElement-frameless.oo-ui-widget-enabled>.oo-ui-buttonElement-button{color:#999}.oo-ui-messageDialog-actions-horizontal .oo-ui-actionWidget{border-right:1px solid #111}.oo-ui-dialog-content>.oo-ui-window-body{outline:1px solid #111}.oo-ui-messageDialog-content>.oo-ui-window-foot{outline:1px solid #111}.wikiEditor-ui .wikiEditor-ui-view{border:1px solid #000}.wikiEditor-ui .wikiEditor-ui-top{border-bottom:1px solid #000}textarea[style]{background:#111!important;color:#fff}.mw-code,pre{background:#222;color:#b0b0b0;border:1px solid #444}.oo-ui-iconWidget.oo-ui-iconElement.oo-ui-iconElement-icon{background-color:#b0b0b0;border:1px solid #333}#trrandomquote{margin:1em 0;padding:15px 35px;background-color:#372c2f;border:1px dashed #251e20;position:relative}#trrandomquote::before{color:#796c22!important}table.tabl1{background:#392c2c3b}#toc,.mw-warning,.toc,ul#filetoc{background:#312424!important;border:solid #582b2b!important}b.b2{background-color:transparent!important;color:#cbae81!important}b.b3{background-color:transparent!important;color:#3c8869!important}.thumbinner{background:0 0!important;border:transparent!important}.num,span.bld{color:#796c22!important}.mw-search-profile-tabs{background:#444!important;border:solid #333!important}.searchmatch{background-color:#daab8bb5!important;color:#000!important}.mw-changeslist-legend{background:0 0!important;color:#ccf!important;border:solid #443f3f!important}fieldset legend{background:#251111!important;color:#3a79b8!important}input[type=submit]:hover{background-color:#703d09}input[type=submit]{background-color:#aa6621}select#namespace{background-color:#393533!important;color:#f0f8ff}.blockquote1{background-color:#292020}