It requires no third-party extensions, the capability for it is right in your settings: Go to your settings -> Advanced Settings
and copy and paste the entire thing in the bottom CSS field
If you still want a really pared-down version, use this
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header,
.main {
color: #333 !important;
background-color: #1b1b1b !important;
}
body,
section,
#page,
.new-comment p,
.related-tags,
.row.notices {
color: #777 !important;
background-color: #1b1b1b !important;
}
nav menu,
footer,
.current a {
color: #777;
background: #272727 !important;
}
.even {
background: #222 !important;
}
.even:hover,
.odd:hover
{
background: #282828 !important;
}
But beware that there are a lot of sections that aren't darkened using this.
Updated