Add support of dark mode

[lk-geimfari]

@media (prefers-color-scheme: dark) {
     body {
         color: #d7d7d8;
         background-color: #161716;
    }
     a {
         color: #66bbff;
    }
     code {
         color: #d5dada;
         background: #313231;
    }
}