A simple and probably incomplete dark theme stylesheet for https://gogs.io sites

gogs-dark.css

:root {
    --fg-color: #EEE;
    --bold-color: #FFF;
    --attached-border-color: #2A2A2A;
    --focus-color: #778;
}

body {
    background-color: #222 !important;
    color: var(--fg-color) !important;
}

/* Headers, footers --------------------------------------------- */

.header-wrapper {
    background-color: #333 !important;
}

.following.bar.light {
    background-color: #222 !important;
    border-bottom-color: #333 !important;
    color: var(--fg-color) !important;
}

.following.bar .top.menu a.item:hover {
    color: rgba(255, 255, 255, 0.45);
}

footer {
    background-color: #333;
    border-color: #222;
}

footer .container .links > * {
    border-color: #666;
}

/* UI things ---------------------------------------------------- */

.ui .text.black {
    color: #AAA;
}

.ui .text.black:hover {
    color: #FFF;
}

.ui .text.grey a {
    color: #888 !important;
}

.ui .text.grey a:hover {
    color: #BBB !important;
}

.ui.header {
    color: #FFF;
}

.ui.breadcrumb .divider {
    color: rgba(255, 255, 255, 0.4);
}

.ui.form .field > label,
.ui.form .inline.field > label, .ui.form .inline.field > p, .ui.form .inline.fields .field > label, .ui.form .inline.fields .field > p, .ui.form .inline.fields > label {
    color: rgba(255, 255, 255, 0.87);
}

.ui.checkbox label, .ui.checkbox + label {
    color: rgba(255, 255, 255, 0.87);
}

.ui.checkbox label:hover, .ui.checkbox + label:hover {
    color: rgba(255, 255, 255, 0.8);
}

.ui.checkbox input:focus ~ .box::after, .ui.checkbox input:focus ~ label, .ui.checkbox input:focus ~ label::after {
    color: rgba(255, 255, 255, 0.95);
}

.ui.input input {
    background: #333;
    color: #CCC;
}

.ui.input input:focus, .ui.input.focus input {
    background: #444;
    color: #AAA;
}

.ui.menu {
    background: #333;
}

.ui.menu .item {
    color: rgba(255, 255, 255, 0.87);
}

.ui.menu .active.item,
.ui.menu .active.item:hover, .ui.vertical.menu .active.item:hover,
.ui.link.menu .item:hover, .ui.menu .dropdown.item:hover, .ui.menu .link.item:hover, .ui.menu a.item:hover,
.ui.menu .ui.dropdown .menu > .active.item {
    background: rgba(255, 255, 255, 0.05) !important;
    color: rgba(255, 255, 255, 0.95) !important;
}

.ui.basic.button, .ui.basic.buttons .button {
    color: rgba(255, 255, 255, 0.6) !important;
    background-color: rgba(230, 230, 230, 0.1) !important;
    border-color: rgba(255, 255, 255, 0.15);
}

.ui.basic.button:focus, .ui.basic.button:hover, .ui.basic.buttons .button:focus, .ui.basic.buttons .button:hover {
    background-color: rgba(180, 180, 180, 0.2) !important;
    color: #FFF !important;
}

.ui.basic.label, .ui.label {
    background-color: rgba(230, 230, 230, 0.1);
    color: #AAA;
}

a.ui.basic.label:hover, a.ui.label:hover, a.ui.labels .label:hover {
    background-color: rgba(180, 180, 180, 0.2);
    color: #FFF;
}

.ui.form textarea,
.ui.form input:not([type]), .ui.form input[type="text"], .ui.form input[type="email"], .ui.form input[type="search"], .ui.form input[type="password"], .ui.form input[type="date"], .ui.form input[type="datetime-local"], .ui.form input[type="tel"], .ui.form input[type="time"], .ui.form input[type="file"], .ui.form input[type="url"], .ui.form input[type="number"] {
    background: #303030;
    color: rgba(255, 255, 255, 0.87);
    border-color: rgba(160, 160, 160, 0.15);
}

.ui.form textarea:focus,
.ui.form input:focus:not([type]), .ui.form input[type="text"]:focus, .ui.form input[type="email"]:focus, .ui.form input[type="search"]:focus, .ui.form input[type="password"]:focus, .ui.form input[type="date"]:focus, .ui.form input[type="datetime-local"]:focus, .ui.form input[type="tel"]:focus, .ui.form input[type="time"]:focus, .ui.form input[type="file"]:focus, .ui.form input[type="url"]:focus, .ui.form input[type="number"]:focus {
    background: #333;
    color: rgba(255, 255, 255, 0.95);
    border-color: var(--focus-color);
}

.ui.attached.header {
    background: #333 !important;
    border-color: var(--attached-border-color) !important;
    color: var(--fg-color) !important;
}

.ui .info.segment.top {
    background-color: #333 !important;
    border-color: #222 !important;
}

.ui.segment {
    background-color: #2A2A2A;
    border-color: #222 !important;
}

.ui.card {
    background: #333;
    color: var(--fg-color) !important;
    border-color: var(--attached-border-color);
}

.ui.card > .content, .ui.cards > .card > .content {
    border-top-color: rgba(255, 255, 255, 0.1) !important;
}

.ui.card > .extra, .ui.cards > .card > .extra {
    border-top-color: rgba(255, 255, 255, 0.05) !important;
}

.ui.card, .ui.cards > .card {
    box-shadow: 0 1px 3px 0 #111, 0 0 0 1px #111;
}

.ui.card > .content > .header, .ui.cards > .card > .content > .header {
    color: rgba(255, 255, 255, 0.85);
}

.ui.card > .extra a:not(.ui), .ui.cards > .card > .extra a:not(.ui) {
    color: rgba(255, 255, 255, 0.4);
}

.ui.table {
    background: #333 !important;
    color: var(--fg-color) !important;
}

.ui.attached.table {
    border-color: var(--attached-border-color);
}

.ui.table thead th {
    color: var(--bold-color);
    background: #444;
}

.ui.tabular.menu {
    border-bottom-color: #444;
}

.ui.tabular.menu .active.item {
    background: #555;
    border-color: #444;
    color: #FFF;
}

.ui.tabular.menu .item {
    color: rgba(255, 255, 255, 0.7);
}

.ui.tabular.menu .item:hover {
    color: rgba(255, 255, 255, 1.0);
}

.ui.secondary.pointing.menu .active.item {
    color: rgba(255, 255, 255, 0.95);
    border-color: #EEE;
}

.ui.secondary.pointing.menu .active.item:hover {
    color: rgba(255, 255, 255, 0.95);
}

.ui.secondary.pointing.menu .dropdown.item:hover, .ui.secondary.pointing.menu .link.item:hover, .ui.secondary.pointing.menu a.item:hover {
    color: rgba(255, 255, 255, 0.87);
}

.ui.secondary.menu .active.item {
    background: rgba(255, 255, 255, 0.05);
    color: rgba(255, 255, 255, 0.95);
}

.ui.secondary.menu .active.item:hover {
    background: rgba(255, 255, 255, 0.04) !important;
}

.ui.menu .dropdown.item .menu,
.ui.dropdown .menu,
.ui.menu, .ui.vertical.menu,
.ui.selection.dropdown {
    background: #333;
    color: var(--fg-color);
}

.ui.dropdown .menu > .header,
.ui.vertical.menu .header.item {
    color: rgba(255, 255, 255, 0.85);
}

.ui.vertical.menu .header.item {
    background: #444;
}

.ui.dropdown .menu .active.item {
    color: rgba(255, 255, 255, 0.95);
}

.ui.dropdown .menu .selected.item, .ui.dropdown.selected {
    background: rgba(255, 255, 255, 0.03);
    color: rgba(255, 255, 255, 0.95);
}

.ui.menu .ui.dropdown .menu > .item:hover, .ui.menu .ui.dropdown .menu > .selected.item {
    background: rgba(255, 255, 255, 0.05) !important;
}

.ui.menu .ui.dropdown .menu > .item,
.ui.dropdown .menu > .item {
    color: rgba(255, 255, 255, 0.87) !important;
}

.ui.secondary.menu .dropdown.item:hover, .ui.secondary.menu .link.item:hover, .ui.secondary.menu a.item:hover {
    color: rgba(255, 255, 255, 0.95);
}

.ui.menu .ui.dropdown .menu > .item:hover, .ui.menu .ui.dropdown .menu > .selected.item {
    color: rgba(255, 255, 255, 0.95) !important;
}

.ui.selection.active.dropdown, .ui.selection.active.dropdown .menu {
    border-color: #333 !important;
}

.ui.selection.dropdown .menu > .item,
.ui.selection.dropdown:focus .menu,
.ui.selection.dropdown:focus {
    border-color: rgba(0, 0, 0, 0.3) !important;
}

.ui.selection.visible.dropdown > .text:not(.default) {
    color: rgba(255, 255, 255, 0.8);
}

.ui.menu .item.disabled, .ui.menu .item.disabled:hover {
    color: rgba(200, 200, 200, 0.3);
}

.ui.menu .item.disabled i.icon {
    color: rgba(200, 200, 200, 0.3);
}

.ui.menu .item > .label {
    background: #555;
    color: #DDD;
}

/* Repository-specific stuff ------------------------------------ */

.repository.file.list #repo-files-table tr:hover {
    background-color: #334258;
}

/* User profile-specific stuff ---------------------------------- */

.user.profile .ui.card .extra.content ul li:not(:last-child) {
    border-bottom: 1px solid #444;
}

/* Issue-specific stuff ----------------------------------------- */

.issue.list > .item .title {
    color: #CCC;
}

.issue.list > .item .title:hover {
    color: #FFF;
}

.repository.view.issue .comment-list .comment .content .header:before,
.repository .comment.form .content .form:before,
.repository .comment.form .content .form:after {
    border-right-color: var(--attached-border-color);
}

.repository.view.issue .comment-list .comment .content .header:after {
    border-right-color: #333;
}

.repository.view.issue .comment-list::before {
    background-color: #333;
}

/* Code stuff --------------------------------------------------- */

.repository .diff-file-box .file-body.file-code .lines-num {
    background: #333 !important;
    border-color: var(--attached-border-color) !important;
}

.repository .diff-file-box .code-diff tbody tr td {
    background: #444 !important;
}

.repository .diff-file-box .code-diff tbody tr.tag-code td {
    background-color: #554 !important;
}

.repository .diff-file-box .code-diff tbody tr.add-code td {
    background: #454 !important;
}

.repository .diff-file-box .code-diff tbody tr.del-code td {
    background: #644 !important;
}

.repository .diff-file-box .code-diff tbody tr.same-code td.active {
    background: #664 !important;
}

.repository .diff-file-box .code-diff tbody tr .added-code {
    background-color: #676;
}

.repository .diff-file-box .code-diff tbody tr .removed-code {
    background-color: #766;
}

.repository.file.list #file-content .code-view .lines-num pre, .repository.file.list #file-content .code-view .lines-code pre, .repository.file.list #file-content .code-view .lines-num ol, .repository.file.list #file-content .code-view .lines-code ol, .repository.file.list #file-content .code-view .lines-num .hljs, .repository.file.list #file-content .code-view .lines-code .hljs {
    background: #444 !important;
}

.repository.file.list #file-content .code-view .lines-num {
    background: #333 !important;
}

.hljs {
    color: #AAA !important;
}

.hljs-keyword, .hljs-selector-tag, .hljs-subst {
    color: #AAA;
}

.hljs-type, .hljs-class .hljs-title {
    color: #ABD;
}

.hljs-number, .hljs-literal, .hljs-variable, .hljs-template-variable, .hljs-tag .hljs-attr {
    color: #7DD;
}

/* Misc. things ------------------------------------------------- */

i.icon {
    color: rgba(255, 255, 255, 0.8);
}

.mega-octicon:before, .octicon:before {
    color: rgba(255, 255, 255, 0.6);
}

Screenshots: