It’s a work in progress, but with this custom CSS using Stylish:
/*
Configure in OGS settings:
board: 131 148 150 (base 0)
lines: 0 43 54 (base03)
black stones: 0 43 54 (base03)
white stones: 238 232 213 (base2)
*/
/* base02 */
.players .black.player-container {
background: #073642 !important;
}
/* base2 */
.players .white.player-container {
background: #eee8d5 !important;
}
/* default text: base0 */
#main-content div {
color: #839496 !important;
}
/* default links: blue */
a {
color: #268bd2 !important;
}
/* navbar links and similar: base1 */
#NavBar a, .Dock a, #NavBar .right .icon-container .fa {
color: #93a1a1 !important;
}
/* default bg: base03 */
#main-content {
background-color: #002b36 !important;
}
/* green highlight for your move: base02 */
.GameList .current-users-move {
background-color: #073642 !important;
}
/* player color: blue */
body .Player {
color: #268bd2 !important;
}
/* chat log bg: base02 */
.chat-log {
background-color: #073642 !important;
}
/* button: base02 on base0 */
button {
color: #002b36 !important;
background-color: #839496 !important;
border-color: #073642 !important;
}
/* leave zen mode: base01 */
.leave-zen-mode-button {
color: #586e75 !important;
}
/* chat presence: base1 / blue */
.ChatPresenceIndicator {
color: #93a1a1 !important;
text-shadow: none !important;
}
.ChatPresenceIndicator.online {
color: #268bd2 !important;
}
/* unread chat: blue */
.ChatIndicator.unread .fa-comment {
color: #268bd2 !important;
}
/* friend indicator: green */
.FriendIndicator.online .fa-users {
color: #859900 !important;
}
/* turn indicator: base02 on base0 */
#NavBar .turn-indicator .count {
color: #002b36 !important;
background-color: #839496 !important;
}
/* shade OGS logo */
.ogs-nav-logo {
background-image: url("data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 425.625 144.375' width='454' height='154'%3E%3Cg fill='%2393a1a1'%3E%3Cpath d='M236.509 1.153c28.915 0 47.145 13.8 57.831 30.073l-24.725 13.183c-6.496-9.681-18.439-17.302-33.106-17.302-25.354 0-43.793 19.156-43.793 45.109 0 25.953 18.439 45.109 43.793 45.109 12.781 0 24.097-5.149 30.173-10.505V90.135h-37.297V65.007h66.632v52.524c-14.249 15.654-34.154 25.953-59.508 25.953-40.859 0-74.385-28.013-74.385-71.269 0-43.461 33.526-71.062 74.385-71.062zM326.392 99.199c9.849 10.093 25.145 18.538 44.421 18.538 16.344 0 24.307-7.621 24.307-15.448 0-10.299-12.153-13.801-28.287-17.509-22.84-5.149-52.174-11.329-52.174-42.02 0-22.863 20.115-41.401 53.012-41.401 22.211 0 40.649 6.591 54.479 19.156l-16.554 21.422C394.281 31.638 379.195 26.9 365.575 26.9c-13.41 0-20.534 5.768-20.534 14.007 0 9.269 11.734 12.153 27.868 15.86 23.049 5.149 52.384 11.947 52.384 42.432 0 25.129-18.229 44.079-55.946 44.079-26.82 0-46.097-8.857-59.088-21.834z'/%3E%3C/g%3E%3Cpath d='M144.33 72.21c0 39.855-32.31 72.165-72.165 72.165C32.31 144.375 0 112.065 0 72.21h144.33z' fill='%23073642'/%3E%3Cpath d='M72.165.045c39.855 0 72.165 32.31 72.165 72.165H0C0 32.355 32.31.045 72.165.045z' fill='%2393a1a1'/%3E%3Cpath d='M9.827 64.743s-.073-.322-.144-.925c-.061-.601-.179-1.497-.206-2.624-.076-2.257.037-5.492.733-9.296.334-1.906.836-3.942 1.473-6.075.675-2.116 1.457-4.344 2.491-6.559a51.579 51.579 0 013.589-6.657c1.385-2.187 2.932-4.337 4.674-6.337.839-1.024 1.791-1.96 2.703-2.916.936-.936 1.93-1.811 2.903-2.683 2.018-1.676 4.112-3.195 6.251-4.477 2.132-1.294 4.307-2.341 6.404-3.204 1.072-.39 2.095-.789 3.103-1.123 1.033-.28 2.016-.563 2.967-.799a42.764 42.764 0 015.262-.868c1.591-.099 2.944-.215 4.086-.155 1.135.035 2.029.137 2.628.176.602.053.923.102.923.102s-.273.193-.785.516c-.515.309-1.255.807-2.199 1.345-.944.543-2.042 1.304-3.307 2.052-.636.367-1.278.832-1.961 1.281-.673.473-1.42.86-2.125 1.413-.719.52-1.475 1.027-2.243 1.563l-2.31 1.737a103.765 103.765 0 00-4.738 3.89c-.8.683-1.565 1.439-2.373 2.148-.787.738-1.537 1.537-2.337 2.283l-2.29 2.4c-.722.85-1.524 1.625-2.222 2.505a69.037 69.037 0 00-2.155 2.579 87.914 87.914 0 00-2.082 2.63l-1.987 2.666c-.616.912-1.274 1.784-1.878 2.679-1.234 1.771-2.376 3.54-3.476 5.231-1.075 1.704-2.114 3.332-3.037 4.873-1.88 3.066-3.431 5.724-4.55 7.614a3466.645 3466.645 0 00-1.785 3.015z' fill='%23eee8d5'/%3E%3Cg%3E%3Cpath d='M84.164 131.848l2.84-2.055c1.779-1.289 4.282-3.079 7.162-5.233 1.449-1.06 2.975-2.245 4.572-3.473 1.583-1.252 3.239-2.551 4.889-3.943.835-.684 1.643-1.419 2.494-2.117l2.472-2.224a89.632 89.632 0 002.427-2.316 69.185 69.185 0 002.37-2.384c.812-.776 1.51-1.647 2.29-2.444l2.179-2.501c.669-.866 1.396-1.687 2.058-2.538.632-.87 1.314-1.701 1.921-2.561 1.234-1.71 2.38-3.413 3.437-5.077l1.516-2.46c.463-.814.898-1.613 1.35-2.377.486-.753.802-1.532 1.211-2.246.384-.722.787-1.404 1.094-2.071.629-1.329 1.285-2.492 1.739-3.482.449-.99.876-1.772 1.136-2.314.274-.54.441-.829.441-.829s.079.316.187.91c.095.593.279 1.473.418 2.601.165 1.131.175 2.49.223 4.083a42.778 42.778 0 01-.379 5.32c-.148.969-.339 1.974-.522 3.029-.24 1.034-.543 2.09-.832 3.193-.667 2.168-1.508 4.43-2.601 6.672-1.08 2.248-2.399 4.474-3.882 6.638-.779 1.05-1.559 2.12-2.404 3.138-.868.995-1.712 2.03-2.654 2.96-1.832 1.919-3.829 3.658-5.88 5.238a51.679 51.679 0 01-6.298 4.187c-2.11 1.234-4.257 2.218-6.301 3.085-2.066.831-4.047 1.518-5.914 2.026-3.724 1.043-6.935 1.454-9.189 1.587-1.126.077-2.028.042-2.632.037a10.034 10.034 0 01-.938-.059z' fill='%23002b36'/%3E%3C/g%3E%3C/svg%3E") !important;
}
Update: maybe I’m getting carried away here, but: