⭐ Merited by bitmover (2)
Tem?
.
Tem algumas extensões que fazem isso.
A maioria usa o Stylus (Chrome/Firefox) que permite adicionar temas via CSS em qualquer site.
No meu caso, eu criei um script que uso no Tampermonkey que adiciona um CSS no site, então mesma coisa, mas sem precisar de outra extensão.
P.S: Nesse caso, peguei o CSS de um usuário (que esqueci o nome) e coloquei no meu próprio script.
A maioria usa o Stylus (Chrome/Firefox) que permite adicionar temas via CSS em qualquer site.
No meu caso, eu criei um script que uso no Tampermonkey que adiciona um CSS no site, então mesma coisa, mas sem precisar de outra extensão.
Code:
// ==UserScript==
// @name BitcoinTalk Dark Mode
// @version 0.1
// @author TryNinja
// @match https://bitcointalk.org/*
// @grant none
// ==/UserScript==
(function() {
'use strict';
var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = `.tborder{border:none;background-color:transparent;padding:0}
form#quickModForm>table.bordercolor>tbody>tr>td{padding:0}
#creator table.tborder,#frmLogin table.tborder,.tborder table#smfheader+table,.tborder table#smfheader+table+table,div#helpmenu+div#helpmain{
border:1px solid #373b44!important}.tborder table#smfheader+table,.tborder table#smfheader+table+table{
border-top:none!important}.tborder table#smfheader+table{border-bottom:none!important}#upshrinkHeader2{
padding-bottom:1px}input,input[type=text],select,textarea{outline:0;padding:4px;transition:all .2s;border-radius:3px}
input:disabled{background-color:#25282f}input[type=button],input[type=submit]{
cursor:pointer;padding:2px 10px!important;transition:all .2s;color:#fff}input:focus,textarea:focus{border-color:#6393bf!important}
input,input[type=text],textarea{border:1px solid #aaa}select{padding:2px}#bodyarea>form>table>tbody>tr>td>table{margin-top:10px}
#bodyarea>form>table>tbody>tr:first-child>td>table{margin-top:0}#bodyarea #quickModForm .bordercolor{background-color:transparent}
#quickModForm #preview_body,#quickModForm .windowbg{background-color:#f7f7f7}#quickModForm .windowbg2{background-color:#fff}
#quickModForm .windowbg,#quickModForm .windowbg2{padding:0;border-radius:5px}form#quickModForm>table:first-child>tbody>tr:first-child td.windowbg:first-child{
border-radius:0 0 5px 5px}#quickModForm td.windowbg2>table>tbody>tr>td+td,#quickModForm td.windowbg>table>tbody>tr>td+td{
padding-right:25px!important;padding-left:25px!important}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info,#quickModForm td.windowbg>table>tbody>tr>td.poster_info{
padding-left:10px;width:14%;overflow:auto!important;border-radius:5px 0 0 5px}#quickModForm td.windowbg>table>tbody>tr>td.poster_info{background-color:#f3f3f3}
#quickModForm td.windowbg2>table>tbody>tr>td.poster_info{background-color:#f7f7f7}.post{padding-top:5px}.hrcolor,hr{color:#d2d2d2;background-color:#d2d2d2}.quote{
color:inherit}.quote,.quoteheader{background-color:transparent;border:none;margin-top:0}.quoteheader{margin-right:3px}.quoteheader:before{
content:url(https://i.ibb.co/W0hX5KP/qicon.png);opacity:.1;margin:0 5px}.quote,.quoteheader{border-left:4px solid #ddd}.quote:hover{
border-left:4px solid #ccc}.quoteheader:hover,.quoteheader:hover+.quote{border-left:4px solid #ccc}div.post[dir=rtl] .quote,div.post[dir=rtl] .quoteheader{
border-right:4px solid #ddd;border-left:0!important}div.post[dir=rtl] .quote:hover{border-right-color:#ccc}div.post[dir=rtl] .quote:hover{
border-right:4px solid #ccc}div.post[dir=rtl] .quoteheader:hover,div.post[dir=rtl] .quoteheader:hover+.quote{border-right:4px solid #ccc}
#bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-image:none;background-color:#5b7e9e}tr.catbg3 #top_subject{
padding-top:5px;padding-bottom:5px}#bodyarea table.tborder tr.catbg3 td:first-child{border-radius:5px 0 0 0}#bodyarea table.tborder tr.catbg3 td:last-child{
border-radius:0 5px 0 0}#bodyarea>div.tborder>table.bordercolor{border-radius:5px 5px 0 0}.catbg,.catbg2,.catbg3{border-bottom:0}
.catbg,.catbg2,.catbg3,tr.catbg td,tr.catbg2 td,tr.catbg3 td{background-image:none;background-color:#5b7e9e}tr.catbg3 #top_subject{
padding-top:5px;padding-bottom:5px}table.bordercolor tr td.catbg3:first-child{border-radius:5px 0 0 0}table.bordercolor tr td.catbg3:last-child{
border-radius:0 5px 0 0}.catbg2:first-child,.tborder>.catbg:first-child,table.bordercolor tr td.catbg:first-child{border-radius:5px 5px 0 0}
.bordercolor{margin-top:0!important}.tborder #smfheader td.catbg:first-child{border-radius:5px 0 0 0}.tborder #smfheader td.catbg:last-child{
border-radius:0 5px 0 0}#bodyarea tr.catbg3+tr td.smalltext{background-color:#fffce5;border-bottom:1px solid #ffde7b}
#quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info form[name=pmFolder] table>tbody>tr>td.windowbg #quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info ,form[name=pmFolder] table.bordercolor>tbody>tr>td.windowbg #upshrinkHeader table>tbody>tr>td.windowbg #quickModForm td.windowbg2>table>tbody>tr>td.poster_info div[style^=overflow],#quickModForm td.windowbg>table>tbody>tr>td.poster_info div[style^=overflow]{
overflow:unset!important}.tborder table#smfheader+table,.tborder table#smfheader+table+table{border:none!important}div[style="border:2px solid orange; background-color:lightyellow; padding:10px; margin-bottom:5px"]{
color:#000}div[style="border:2px solid red; background-color:#FF9696; padding:10px; margin-bottom:5px"]{color:#000}div[style="border:2px solid red; background-color:#FF9696; padding:10px; margin-bottom:5px"] a{color:#666}
input,textarea{border-color:#424448!important;background-color:#191c23}
input:focus,textarea:focus{border-color:#567ee0!important}input[type=button],input[type=submit]{border:none;background-color:#3f51b5;border-radius:3px;padding:4px 7px!important}input[type=button]:hover,input[type=submit]:hover{background-color:#5265d9}
.titlebg,.titlebg2,tr.titlebg td,tr.titlebg th,tr.titlebg2 td,tr.titlebg2 th{border:none;background-image:none;background-color:#373b44}
#preview_body,.code,.codeheader,.nav,.nav:link,.nav:visited,.quoteheader,.titlebg,.titlebg a:link,.titlebg a:visited,.titlebg2,.titlebg2 a:link,.titlebg2 a:visited,.windowbg,.windowbg2,.windowbg3,a.nav:hover,body,input,td,textarea,th,tr,tr.titlebg td,tr.titlebg th,tr.titlebg2 td,tr.titlebg2 th{color:#ccc}#preview_body,.windowbg{background-color:#25282f}a:visited{color:#939dc5}
a:link{color:#939dc5}a.ul[href^="https://bitcointalk.org/"]:hover{color:#89c775!important}.edited{border-bottom-color:#d6d6d6}div#bodyarea .maintab_back a:link,div#bodyarea .maintab_back a:visited,div#bodyarea .mirrortab_back a:link,div#bodyarea .mirrortab_back a:visited{color:#939dc5}#quickModForm #preview_body,#quickModForm .windowbg,#quickModForm .windowbg2{background-color:#23262f}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info,#quickModForm td.windowbg>table>tbody>tr>td.poster_info{background-color:#20232b}.hrcolor,hr{
color:#393c44;background-color:#393c44}.quoteheader,.quoteheader a{color:#737c9e}.quote,.quoteheader{border-left:4px solid #3d444e}.quote:hover{border-left:4px solid #62676f}.quoteheader:hover,.quoteheader:hover+.quote{border-left:4px solid #62676f}div.post[dir=rtl] .quote,div.post[dir=rtl] .quoteheader{border-right:4px solid #3d444e}
div.post[dir=rtl] .quote:hover{border-right-color:#62676f}div.post[dir=rtl] .quote:hover{border-right:4px solid #62676f}div.post[dir=rtl] .quoteheader:hover,div.post[dir=rtl] .quoteheader:hover+.quote{border-right:4px solid #62676f}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info form[name=pmFolder] table>tbody>tr>td.windowbg #bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-color:#373b44}.catbg,.catbg2,.catbg3,tr.catbg td,tr.catbg2 td,tr.catbg3 td{
background-color:#3f4554}#bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-color:#373b44}#bodyarea tr.catbg3+tr td.smalltext{
background-color:#292d38;border-bottom-color:#373b44;color:#fff}[style^="color: black"],[style^="color:black"]{color:#d6d6d6!important}[style^="color: green"],[style^="color:green"]{
color:#5bb55b!important}[style^="color: red"],[style^="color:red"]{color:#ff3434!important}[style^="color: blue"],[style^="color:blue"]{color:#1f81ff!important
}[style^="background-color: yellow"],[style^="background-color:yellow"]{color:#000}.titlebg2{background-image:none;border:none;background-color:#2d313a;color:#d6d6d6}
img.reply_button{content:url(https://i.imgur.com/ZE3fDNe.png)}img.modify_button{content:url(https://i.imgur.com/AmesFHS.png)}img.remove_button{
content:url(https://i.imgur.com/vTEXCG8.png)}.quoteheader:before{content:url(https://i.ibb.co/QpYLk0v/qiconw.png)!important;opacity:.22}.code{
background-color:#2f323c;border-color:#444850}.windowbg2{background-color:#262931}.bordercolor{background-color:#363940}body{background-color:#191c23;color:#fff}
.windowbg3{background-color:#22242b}.maintab_back{background-image:url(https://i.ibb.co/v1vrm0d/maintab-back.gif)}
.maintab_first{background-image:url(https://i.ibb.co/DKK4rrX/maintab-first.gif)}.maintab_last{
background-image:url(https://i.ibb.co/C9QxkDg/maintab-last.gif)}.maintab_active_first{background-image:url(https://i.ibb.co/Q96FRdx/maintab-active-first.gif)}
.maintab_active_back{background-image:url(https://i.ibb.co/5Tk3s5j/maintab-active-back.gif)}.maintab_active_last{background-image:url(https://i.ibb.co/drDZN4d/maintab-active-last.gif)}
::-webkit-scrollbar{width:10px;height:8px}::-webkit-scrollbar-track-piece{background-color:#23262f;-webkit-border-radius:6px}::-webkit-scrollbar-thumb:vertical{
height:50px;background-color:#373b44;-webkit-border-radius:6px}::-webkit-scrollbar-thumb:horizontal{width:50px;background-color:#373b44;-webkit-border-radius:3px}
.code::-webkit-scrollbar{width:7px;height:7px}.code::-webkit-scrollbar-thumb:vertical{height:50px;background-color:#13161c;-webkit-border-radius:6px}
.code::-webkit-scrollbar-track-piece{background-color:#23262f;-webkit-border-radius:6px}.windowbg b a{color:#fff}`;
document.getElementsByTagName('head')[0].appendChild(style);
})();
// @name BitcoinTalk Dark Mode
// @version 0.1
// @author TryNinja
// @match https://bitcointalk.org/*
// @grant none
// ==/UserScript==
(function() {
'use strict';
var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = `.tborder{border:none;background-color:transparent;padding:0}
form#quickModForm>table.bordercolor>tbody>tr>td{padding:0}
#creator table.tborder,#frmLogin table.tborder,.tborder table#smfheader+table,.tborder table#smfheader+table+table,div#helpmenu+div#helpmain{
border:1px solid #373b44!important}.tborder table#smfheader+table,.tborder table#smfheader+table+table{
border-top:none!important}.tborder table#smfheader+table{border-bottom:none!important}#upshrinkHeader2{
padding-bottom:1px}input,input[type=text],select,textarea{outline:0;padding:4px;transition:all .2s;border-radius:3px}
input:disabled{background-color:#25282f}input[type=button],input[type=submit]{
cursor:pointer;padding:2px 10px!important;transition:all .2s;color:#fff}input:focus,textarea:focus{border-color:#6393bf!important}
input,input[type=text],textarea{border:1px solid #aaa}select{padding:2px}#bodyarea>form>table>tbody>tr>td>table{margin-top:10px}
#bodyarea>form>table>tbody>tr:first-child>td>table{margin-top:0}#bodyarea #quickModForm .bordercolor{background-color:transparent}
#quickModForm #preview_body,#quickModForm .windowbg{background-color:#f7f7f7}#quickModForm .windowbg2{background-color:#fff}
#quickModForm .windowbg,#quickModForm .windowbg2{padding:0;border-radius:5px}form#quickModForm>table:first-child>tbody>tr:first-child td.windowbg:first-child{
border-radius:0 0 5px 5px}#quickModForm td.windowbg2>table>tbody>tr>td+td,#quickModForm td.windowbg>table>tbody>tr>td+td{
padding-right:25px!important;padding-left:25px!important}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info,#quickModForm td.windowbg>table>tbody>tr>td.poster_info{
padding-left:10px;width:14%;overflow:auto!important;border-radius:5px 0 0 5px}#quickModForm td.windowbg>table>tbody>tr>td.poster_info{background-color:#f3f3f3}
#quickModForm td.windowbg2>table>tbody>tr>td.poster_info{background-color:#f7f7f7}.post{padding-top:5px}.hrcolor,hr{color:#d2d2d2;background-color:#d2d2d2}.quote{
color:inherit}.quote,.quoteheader{background-color:transparent;border:none;margin-top:0}.quoteheader{margin-right:3px}.quoteheader:before{
content:url(https://i.ibb.co/W0hX5KP/qicon.png);opacity:.1;margin:0 5px}.quote,.quoteheader{border-left:4px solid #ddd}.quote:hover{
border-left:4px solid #ccc}.quoteheader:hover,.quoteheader:hover+.quote{border-left:4px solid #ccc}div.post[dir=rtl] .quote,div.post[dir=rtl] .quoteheader{
border-right:4px solid #ddd;border-left:0!important}div.post[dir=rtl] .quote:hover{border-right-color:#ccc}div.post[dir=rtl] .quote:hover{
border-right:4px solid #ccc}div.post[dir=rtl] .quoteheader:hover,div.post[dir=rtl] .quoteheader:hover+.quote{border-right:4px solid #ccc}
#bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-image:none;background-color:#5b7e9e}tr.catbg3 #top_subject{
padding-top:5px;padding-bottom:5px}#bodyarea table.tborder tr.catbg3 td:first-child{border-radius:5px 0 0 0}#bodyarea table.tborder tr.catbg3 td:last-child{
border-radius:0 5px 0 0}#bodyarea>div.tborder>table.bordercolor{border-radius:5px 5px 0 0}.catbg,.catbg2,.catbg3{border-bottom:0}
.catbg,.catbg2,.catbg3,tr.catbg td,tr.catbg2 td,tr.catbg3 td{background-image:none;background-color:#5b7e9e}tr.catbg3 #top_subject{
padding-top:5px;padding-bottom:5px}table.bordercolor tr td.catbg3:first-child{border-radius:5px 0 0 0}table.bordercolor tr td.catbg3:last-child{
border-radius:0 5px 0 0}.catbg2:first-child,.tborder>.catbg:first-child,table.bordercolor tr td.catbg:first-child{border-radius:5px 5px 0 0}
.bordercolor{margin-top:0!important}.tborder #smfheader td.catbg:first-child{border-radius:5px 0 0 0}.tborder #smfheader td.catbg:last-child{
border-radius:0 5px 0 0}#bodyarea tr.catbg3+tr td.smalltext{background-color:#fffce5;border-bottom:1px solid #ffde7b}
#quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info form[name=pmFolder] table>tbody>tr>td.windowbg #quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info ,form[name=pmFolder] table.bordercolor>tbody>tr>td.windowbg #upshrinkHeader table>tbody>tr>td.windowbg #quickModForm td.windowbg2>table>tbody>tr>td.poster_info div[style^=overflow],#quickModForm td.windowbg>table>tbody>tr>td.poster_info div[style^=overflow]{
overflow:unset!important}.tborder table#smfheader+table,.tborder table#smfheader+table+table{border:none!important}div[style="border:2px solid orange; background-color:lightyellow; padding:10px; margin-bottom:5px"]{
color:#000}div[style="border:2px solid red; background-color:#FF9696; padding:10px; margin-bottom:5px"]{color:#000}div[style="border:2px solid red; background-color:#FF9696; padding:10px; margin-bottom:5px"] a{color:#666}
input,textarea{border-color:#424448!important;background-color:#191c23}
input:focus,textarea:focus{border-color:#567ee0!important}input[type=button],input[type=submit]{border:none;background-color:#3f51b5;border-radius:3px;padding:4px 7px!important}input[type=button]:hover,input[type=submit]:hover{background-color:#5265d9}
.titlebg,.titlebg2,tr.titlebg td,tr.titlebg th,tr.titlebg2 td,tr.titlebg2 th{border:none;background-image:none;background-color:#373b44}
#preview_body,.code,.codeheader,.nav,.nav:link,.nav:visited,.quoteheader,.titlebg,.titlebg a:link,.titlebg a:visited,.titlebg2,.titlebg2 a:link,.titlebg2 a:visited,.windowbg,.windowbg2,.windowbg3,a.nav:hover,body,input,td,textarea,th,tr,tr.titlebg td,tr.titlebg th,tr.titlebg2 td,tr.titlebg2 th{color:#ccc}#preview_body,.windowbg{background-color:#25282f}a:visited{color:#939dc5}
a:link{color:#939dc5}a.ul[href^="https://bitcointalk.org/"]:hover{color:#89c775!important}.edited{border-bottom-color:#d6d6d6}div#bodyarea .maintab_back a:link,div#bodyarea .maintab_back a:visited,div#bodyarea .mirrortab_back a:link,div#bodyarea .mirrortab_back a:visited{color:#939dc5}#quickModForm #preview_body,#quickModForm .windowbg,#quickModForm .windowbg2{background-color:#23262f}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info,#quickModForm td.windowbg>table>tbody>tr>td.poster_info{background-color:#20232b}.hrcolor,hr{
color:#393c44;background-color:#393c44}.quoteheader,.quoteheader a{color:#737c9e}.quote,.quoteheader{border-left:4px solid #3d444e}.quote:hover{border-left:4px solid #62676f}.quoteheader:hover,.quoteheader:hover+.quote{border-left:4px solid #62676f}div.post[dir=rtl] .quote,div.post[dir=rtl] .quoteheader{border-right:4px solid #3d444e}
div.post[dir=rtl] .quote:hover{border-right-color:#62676f}div.post[dir=rtl] .quote:hover{border-right:4px solid #62676f}div.post[dir=rtl] .quoteheader:hover,div.post[dir=rtl] .quoteheader:hover+.quote{border-right:4px solid #62676f}#quickModForm td.windowbg2>table>tbody>tr>td.poster_info #quickModForm td.windowbg>table>tbody>tr>td.poster_info form[name=pmFolder] table>tbody>tr>td.windowbg #bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-color:#373b44}.catbg,.catbg2,.catbg3,tr.catbg td,tr.catbg2 td,tr.catbg3 td{
background-color:#3f4554}#bodyarea table.tborder tr.catbg3,#bodyarea table.tborder tr.catbg3 td{background-color:#373b44}#bodyarea tr.catbg3+tr td.smalltext{
background-color:#292d38;border-bottom-color:#373b44;color:#fff}[style^="color: black"],[style^="color:black"]{color:#d6d6d6!important}[style^="color: green"],[style^="color:green"]{
color:#5bb55b!important}[style^="color: red"],[style^="color:red"]{color:#ff3434!important}[style^="color: blue"],[style^="color:blue"]{color:#1f81ff!important
}[style^="background-color: yellow"],[style^="background-color:yellow"]{color:#000}.titlebg2{background-image:none;border:none;background-color:#2d313a;color:#d6d6d6}
img.reply_button{content:url(https://i.imgur.com/ZE3fDNe.png)}img.modify_button{content:url(https://i.imgur.com/AmesFHS.png)}img.remove_button{
content:url(https://i.imgur.com/vTEXCG8.png)}.quoteheader:before{content:url(https://i.ibb.co/QpYLk0v/qiconw.png)!important;opacity:.22}.code{
background-color:#2f323c;border-color:#444850}.windowbg2{background-color:#262931}.bordercolor{background-color:#363940}body{background-color:#191c23;color:#fff}
.windowbg3{background-color:#22242b}.maintab_back{background-image:url(https://i.ibb.co/v1vrm0d/maintab-back.gif)}
.maintab_first{background-image:url(https://i.ibb.co/DKK4rrX/maintab-first.gif)}.maintab_last{
background-image:url(https://i.ibb.co/C9QxkDg/maintab-last.gif)}.maintab_active_first{background-image:url(https://i.ibb.co/Q96FRdx/maintab-active-first.gif)}
.maintab_active_back{background-image:url(https://i.ibb.co/5Tk3s5j/maintab-active-back.gif)}.maintab_active_last{background-image:url(https://i.ibb.co/drDZN4d/maintab-active-last.gif)}
::-webkit-scrollbar{width:10px;height:8px}::-webkit-scrollbar-track-piece{background-color:#23262f;-webkit-border-radius:6px}::-webkit-scrollbar-thumb:vertical{
height:50px;background-color:#373b44;-webkit-border-radius:6px}::-webkit-scrollbar-thumb:horizontal{width:50px;background-color:#373b44;-webkit-border-radius:3px}
.code::-webkit-scrollbar{width:7px;height:7px}.code::-webkit-scrollbar-thumb:vertical{height:50px;background-color:#13161c;-webkit-border-radius:6px}
.code::-webkit-scrollbar-track-piece{background-color:#23262f;-webkit-border-radius:6px}.windowbg b a{color:#fff}`;
document.getElementsByTagName('head')[0].appendChild(style);
})();
P.S: Nesse caso, peguei o CSS de um usuário (que esqueci o nome) e coloquei no meu próprio script.
e
Sobre a pergunta, é exatamente isso que o ninja postou. No meu caso eu uso o Stylish que, no final das contas, foi feito por um user daqui. Eu sou muito mais adepto do dark mode, seja onde for, amor aos meus olhos - que já não são aquelas coisas 
Recomendo que troque o Sylish pelo Stylus. Se não me engano, o Stylish já foi banida do Chrome/Firefox e Opera por analisar, salvar e enviar à seus servidores, todos os sites que seus usuários visitavam[1].Assim, foi criado o Sylus, um fork da Sylish que respeita sua privacidade. A funcionalidade é a mesma.
[1] https://arstechnica.com/information-technology/2018/07/stylish-extension-with-2m-downloads-banished-for-tracking-every-site-visit/