YouTrack dark theme. Set Darcula theme first!

dark.css

body, .darcula .sb-row, .darcula .sb-row, .darcula .sb-board-title {
    background: #303030 !important;
    color: #eeeeee !important;
}

.yt-header__create-btn > .ring-menu__item__i {
    background: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#ef9331), to(#e9701d)) !important;
}

.ring-menu, .jt-menu-button, .ring-dropdown__item, .suggest, .inputWrapper > input {
    background: #303030 !important;
    color: #eeeeee;
}

.darcula .sb-task-estimate {
    background-color: #595959 !important;
    color: white !important;
}

.search-center, .search-west,.search-panel {
    background: #303030 !important;
}

.selectProject .arrow {
    url("/_classpath/smartui/img/default/context-arrow-h-7d62093c.png") no-repeat 8px 12px !important;
}

.selectProject {
    box-shadow: #303030 0px 1px 0px 0px !important;
}

.selectProject .inputWrapper {
    padding-right: 0px;
}

.suggest ul li.current {
    background-color: #595959;
}

.ring-dropdown__i, .comboboxList {
    background: #303030 !important;
}

.ring-menu__item__i {
    background: #303030 !important;
    color: #eeeeee !important;
}

.sb-sprint-progress, .sb-sprint-version {
    color: #eeeeee !important;
}

.sb-task-summary, .darcula .sb-task-in {
    background: #424242 !important;
    color: #eeeeee !important;
}

.darcula .toolbar-unpined {
    background: #303030 !important;
    color: #eeeeee !important;
}

.darcula .sb-column-issue-list-link {
    color: #eeeeee !important;
}

.darcula .sb-column-issue-list-link span {
    color: #eeeeee !important;
}

.darcula .sb-board-name, .sb-column-titles {
    background: #303030 !important;
    color: #eeeeee !important;
}

.sb-c16, .c16 {
    background-color: #8CB9A4 !important;
    color: #eeeeee !important;
    background-image: none;
}

.sb-c17, .c17 {
    background-color: #34b67a !important;
    color: #eeeeee !important;
}

.sb-c18, .c18 {
    background-color: #FF7A7A !important;
    color: #eeeeee !important;
}

.sb-c19, .c19 {
    background-color: #B84949 !important;
    color: #eeeeee !important;
}

.sb-task-estimate {
    color: #eeeeee !important;
}

.attribute.c19, .attribute-color-mark.c19, .Enumerations .c19, .colorsTable .c19, .jt-prop-title.c19, .sb-task-summary.c19 {
    background-color: #B84949 !important;
    color: #eeeeee !important;
}

.search-panel-input > input {
    background: #595959 !important;
    color: #eeeeee !important;
}

.search-center .panel {
    border-color: #303030 !important;
    border-bottom-color: #303030 !important;
    box-shadow: #303030 1px 2px 2px -2px inset, #303030 0px 1px 0px 0px !important;
}

body, .ui-widget {
    font-family: "Segoe UI", "Helvetica Neue", arial, sans-serif;
}

.sb-issue-edit-dlg-content {
    background: #595959 !important;
    color: #eeeeee !important;
}

.jt-dialog, .jt-bl-wrapper {
    background: #595959 !important;
    color: #eeeeee !important;
}

.sb-issue-edit-description, .sb-issue-edit-summary, .sb-jeditable input, .sb-issue-edit-south, .sb-issue-edit-addcomment-input, .sb-issue-edit__fields-container td {
    color: #eeeeee !important;
    background: #595959 !important;
}

#editIssueDialog a, a:visited {
    color: #FFFFFF !important;
    text-decoration: none;
}

Any advice how to use this theme?

I would use a tool like Stylebot or Stylish to apply it to your page @BitMatrix or @truonghatsts, although I would describe this as a good start but significantly out of date or incomplete.... but someone with decent CSS skills should be able to whip it into shape in 20-30 minutes, I think.