Add this CSS via the "User CSS" Chrome extension: https://chrome.google.com/webstore/detail/user-css/okpjlejfhacmgjkmknjhadmkdbcldfcb?hl=en

youtrack-agile-board-darcula

.yt-agile-table__row__cell {
 	border-color: #232e34;
  border-right: 1px solid #232e34;
  border-left: 1px solid #232e34;
}

.yt-agile-table .yt-agile-table__row {
  border-bottom: 1px solid #232e34;
}

.yt-sticky-panel__container_pinned .yt-agile-table__row__cell_head:last-child {
  border: 1px solid #232e34;  
}

.yt-agile-table {
	background-color: #394c55;
}

.yt-agile-board__toolbar__slider-wrapper, .yt-sticky-panel__container_pinned .yt-agile-board__toolbar__slider-wrapper {
  background: #394c55;
}

.yt-agile-table__row-container__head_sticky {
  background-color: #394c55;
}

.global .yt-agile-board {
	color: #fff;
}

.ring-island, .yt-agile-card {
	color: #fff;
	background-color: #232e34;
	border: none;
}

.yt-agile-card.yt-agile-card_selected {
  background-color: #355559;
}

.global .yt-agile-table__row-title__summary, .global .yt-agile-table__row__cell, .yt-agile-card__summary, .yt-drag-agile-card__summary {
	color: #fff;
}

.global .yt-dark-grey-text {
	color: #25b7ff;
}

.yt-agile-board__grey-link {
	color: #25b7ff;
}

.global .yt-agile-table__row__estimation.yt-dark-grey-text {
  color:#888;
	background-color: #232e34;
}

.yt-agile-table__row_orphan_white {
  background-color: #394c55;
}

yt-agile-bottom-toolbar {
  display:none;
}

.yt-agile-board.yt-page {
  background-color: #394c55;
  border-top: 3px solid #232e34;
  border-bottom: 3px solid #232e34;
}

.yt-sticky-panel__container {
  background-color: #394c55;
}

.yt-agile-board__toolbar__sprint {
  background-image: none;
  background-color: #232e34;
  color: #fff;
  box-shadow:none;
}

.yt-page__block .ring-input, 
.yt-page__block .ring-input-size_l {
  background-color: #232e34;
  color: #fff;
  border: none;
  box-shadow:none;
}

.yt-page__block .ring-tag {
    background-color: #394c55;
    border:none;
}

.yt-page__block .yt-search-panel__input .ring-icon_gray {
  background-color: #232e34;
  color: #fff;
}

.yt-page__block .ring-button:not([disabled]):not(.ring-button_loader),
.yt-page__block .ring-button.ring-button_active:not([disabled]):not(.ring-button_loader) {
  background-color: #232e34;
  background-image: none;
  color:#fff;
}
.yt-page__block .ring-button.ring-button_blue,
.yt-page__block .ring-button.ring-button_blue:hover {
  background-color: #232e34;
  background-image: none;
  background: none;
  box-shadow:none;
}

.yt-page__block .yt-column-settings .ring-table__header {
    background-color: #394c55;
}

.yt-page__block .ring-table__column {
  background-color: #394c55;
}

.yt-page__block .ring-link {
	color: #25b7ff;
}

.yt-page__block .ring-tabs__btn {
  color:#fff;
}

.yt-agile-table__row {
  border-bottom: none
}

.ring-query-assist {
  background: none;
}

.yt-page__block  .ring-query-assist__letter, .global.background__gradient.global_agile-board {
    color: white;
}

.yt-sticky-panel__container.yt-sticky-panel__container_pinned.yt-agile-table__row-container__head_sticky, 
.yt-sticky-panel__container.yt-sticky-panel__container_pinned.yt-agile-board__toolbar_sticky {
  visibility: hidden;
}

.global .yt-issue-comment__text, .global .yt-issue-key-value-list__column_key, .global .command-dialog-container.command-dialog-container {
  color: #444;
}

Nice css. I had an issue with the total estimate background color.

I added this css code.
.yt-agile-table__row__estimation {
background-color: #232e34;
}