Archive of Our Own

Colours and Images used:

Background- https://i.imgur.com/82Cf5wF.png
#161616- near-black that's the same as the background of the image
#eee- near-white that is used for text
#768e99- grey-blue colour used for borders and buttons and titles
#334c74- colour for text when hovered over
#393374- colour for visited pages

If you wanna replace the image, you've got to change the bit that says background-image: url("https://i.imgur.com/82Cf5wF.png"); with background: # and input the hex code of the colour you want or vice versa
Also, note that colour is spelt without the u
Also also, I've added comments to the code which are inside the /* */ that say what the bits of code change

Here's how it looks

CSS code

#outer .region,
#footer .group,
.post fieldset fieldset,
fieldset fieldset {
background-image: url("https://i.imgur.com/82Cf5wF.png");
}

body,
.group,
.group .group,
.region,
.flash,
fieldset,
fieldset fieldset ul,
form dl,
textarea,
#main .verbose legend,
.verbose fieldset,
.notice,
ul.notes,
input,
textarea,
table,
th,
td:hover,
tr:hover,
.symbol .question:hover,
#modal,
.ui-sortable li,
.required .autocomplete,
.autocomplete .notice,
.system .intro,
.comment_error,
.kudos_error,
div.dynamic,
.dynamic form,
#ui-datepicker-div,
.ui-datepicker table {
background: #161616;
color: #eee; /* text color of works, does not include tags or anything else I think */
border-color: #768e99; /* border around options (sort and filter, etc) */
outline: #111;
box-shadow: none;
}

#header .actions a:hover,
#header .actions a:focus,
#header .dropdown:hover a,
#header .open a,
#header .menu,
#small_login,
.group.listbox,
fieldset fieldset.listbox,
form blockquote.userstuff,
input:focus,
textarea:focus,
li.relationships a,
.group.listbox .index,
.dashboard fieldset fieldset.listbox .index,
#dashboard a:hover,
th,
#dashboard .secondary,
.secondary,
.thread .even,
.system .tweet_list li,
.ui-datepicker tr:hover {
background: #161616;
}

#header .dropdown .menu a:hover,
#header .dropdown .menu a:focus,
.splash .favorite li:nth-of-type(odd) a,
.ui-datepicker td:hover,
#tos_prompt .heading,
#tos_prompt [disabled] {
background: #161616;
color: #334c74;
}

#outer,
.javascript,
.statistics .index li:nth-of-type(even),
#tos_prompt,
.announcement input[type="submit"] {
background: #161616;
}

#header ul.primary,
#outer #footer,
.toggled form {
background-image: url("https://i.imgur.com/82Cf5wF.png");
}

#header ul.primary,
#footer,
#dashboard ul,
dl.meta,
.group.listbox,
fieldset fieldset.listbox,
#main li.blurb,
form blockquote.userstuff,
div.comment,
li.comment,
.toggled form,
form dl dt,
form.single fieldset,
#inner .module .heading,
.bookmark .status span,
.splash .news li,
.filters .group dt.bookmarker {
border-color: #768e99; /* border around works */
}

.group.listbox,
fieldset fieldset.listbox,
#main li.blurb,
.wrapper,
#dashboard .secondary,
.secondary,
form blockquote.userstuff,
.thread .comment,
.toggled form {
box-shadow: none;
}

#dashboard .current,
.actions a:active,
#outer .current,
a.current,
.current a:visited,
span.unread,
.replied,
span.claimed,
dl.index dd,
.own,
.draft,
.draft .unread,
.child,
.unwrangled,
.unreviewed,
.ui-sortable li:hover {
background: #000;
border-color: #768e99;
box-shadow: none;
}

input,
textarea {
box-shadow: none;
}

li.blurb,
.blurb .blurb,
.listbox .index,
fieldset fieldset.listbox,
.dashboard .listbox .index {
box-shadow: none;
}

#footer a:hover,
#footer a:focus,
.autocomplete .dropdown ul li:hover,
.autocomplete .dropdown li.selected,
a.tag:hover,
.listbox .heading a.tag:visited:hover,
.symbol .question,
.qtip-content {
background: #161616;
color: #334c74;
}

.splash .favorite li:nth-of-type(odd) a:hover,
.splash .favorite li:nth-of-type(odd) a:focus {
background: #161616;
color: #111;
}

#header #greeting img,
#header .heading a,
#header .heading a:visited,
#header .user a:hover,
#header .user a:focus,
#header fieldset,
#header form,
#header p,
#dashboard a:hover,
.actions a:hover,
.actions input:hover,
.delete a,
span.delete,
span.unread,
.replied,
span.claimed,
.draggable,
.droppable,
span.requested,
a.work,
.blurb h4 a:link,
.blurb h4 img,
.splash .module h3,
.splash .browse li a:before,
.required,
.error,
.comment_error,
.kudos_error,
a.cloud7,
a.cloud8,
#tos_prompt .heading {
color: #768e99;
}

#greeting .icon,
#dashboard,
#dashboard.own,
.error,
.comment_error,
.kudos_error,
.LV_invalid,
.LV_invalid_field,
input.LV_invalid_field:hover,
input.LV_invalid_field:active,
textarea.LV_invalid_field:hover,
textarea.LV_invalid_field:active,
.qtip-content {
border-color: #161616;
}

a,
a:link,
a.tag,
#header a,
#header a:visited,
#header .primary .open a,
#header .primary .dropdown:hover a,
#header .primary .dropdown a:focus,
#header #search input:focus,
#header #search input:hover,
#dashboard a,
#dashboard span,
#dashboard .current,
.heading,
.group .heading,
.filters dt a:hover {
color: #eee;
}

a:visited,
.actions a:visited,
.action a:link,
.action a:visited,
.listbox .heading a:visited,
span.series .divider {
color: #393374;
}

.actions a,
.actions a:link,
.action,
.action:link,
.actions input,
input[type="submit"],
button,
.current,
.actions label,
#header .actions a {
background: #768e99; /* bg of search bar, fandoms, browse, search and about buttons */
border-color: #111;
color: #eee;
box-shadow: none;
text-shadow: none;
}

.actions a:hover,
.actions input:hover,
#dashboard a:hover,
.actions a:focus,
.actions input:focus,
#dashboard a:focus {
color: #334c74; /* text in buttons when hovered over, doesn’t include buttons on top */
border-color: #000;
box-shadow: none;
}

.actions a:active,
.current,
a.current,
.current a:visited {
color: #eee;
background: #768e99;
border-color: none;
box-shadow: none;
}

.delete a,
span.delete {
box-shadow: none;
}

ul.required-tags,
.bookmark .status span,
.blurb .icon {
opacity: 0.9;
border: 0;
}

#outer .group .heading,
#header .actions a,
fieldset.listbox .heading,
.userstuff .heading,
.heading,
.userstuff h2 {
text-shadow: none;
color: #eee;
background: 768e99;
}

#header .actions a,
fieldset fieldset,
.mce-container button,
.filters .expander {
box-shadow: 768e99;
}

fieldset fieldset.listbox {
outline: none;
}

form dd.required {
color: #eee;
}

.mce-container input:focus {
background: #F3EFEC;
}

.announcement .userstuff a,
.announcement .userstuff a:link,
.announcement .userstuff a:visited:hover {
color: #111;
}

.announcement .userstuff a:visited {
color: #334c74;
}

.announcement .userstuff a:hover,
.announcement .userstuff a:focus {
color: #334c74;
}

.event.announcement .userstuff a,
.filters .expander {
color: #eee;
}