https://www.reddit.com/r/Slack/comments/5x6uf4/dark_theme_for_the_desktop_app/df2dr5u/

slack-custom.css

:root {
   --primary: #09F;
   --accent: #57A;

   --background: #080808;
   --background-elevated: #222;
   --background-hover: rgba(255, 255, 255, 0.1);
   --sidebar-background: #1a2129;

   --border-dim: #666;
   --border-bright: var(--primary);

   --text: #CCC;
   --text-alt: var(--accent);
   --text-special: var(--primary);

   --scrollbar-background: #000;
   --scrollbar-border: var(--primary);
}

#msgs_scroller_div::-webkit-scrollbar-track, #client_body::before, .client_container,
#search_terms, #client_body, #footer, ts-message, .channel_header, ts-jumper ts-jumper-container,
ts-jumper input[type="text"],
.supports_custom_scrollbar:not(.slim_scrollbar) #col_channels:hover #channels_scroller::-webkit-scrollbar-track{
  background: var(--background)  !important;
}

#client_body::before {
  border-bottom: 1px solid var(--background) !important;
}

ts-message, .channel_title .channel_name, ts-jumper input[type="text"],
ts-jumper ol li .member_real_name, ts-jumper ol li .view_name, ts-jumper ol li .channel_name {
  color: #eee !important;
}

.light_theme ts-message .message_content a.message_sender, #msg_input::-webkit-input-placeholder,
ts-jumper input[type="text"]::-webkit-input-placeholder {
  color: #aaa !important;
}

#msg_input, #primary_file_button {
  background: transparent !important;
  color: var(--text) !important;
  border-color: var(--border-dim) !important;
}

.day_container .day_divider .day_divider_label, .day_divider .day_divider_label {
  color: var(--text) !important;
  background-color: var(--background-elevated) !important;
  border: 1px solid var(--border-bright) !important;
}

.mention_day_container_div .day_divider:before {
   background-color: transparent !important;
}

.day_container .day_msgs {
  border-top: 1px solid var(--border-dim) !important;
}

ts-message {
   border-radius: 5px;
   margin: 1px !important;
   border-right: 1px;
   border-left: 1px;
   1px solid transparent !important; /* reserve space for hover */
}

ts-message:hover {
   margin: 0px !important;
   background: var(--background-hover) !important;
   border: 1px solid var(--border-bright) !important
}

#messages_container::after {
   background: none !important;
}

.bot_label {
   padding: 0 4px !important;
   border-radius: 3px !important;
   background: var(--background) !important;
   border: 1px solid var(--border-bright);
}


.file_container:after{
   background-image: -webkit-linear-gradient(top,rgba(255,255,255,0) 0,#333 100%) !important;
}

.rxn,
#team_menu,
#col_channels,
#col_channels_footer,
pre,
ts-message .action_hover_container .btn_msg_action,
#messages_container.has_top_messages_banner:before,
.file_container,
.file_container .CodeMirror .CodeMirror-code>div:before, .file_container .CodeMirror .sssh-line:before, .file_container .sssh-code .CodeMirror-code>div:before, .file_container .sssh-code .sssh-line:before,
#col_flex,
.search_message_result,
#flex_contents .heading,
#flex_contents .heading a,
#flex_contents .subheading,
.search_result_with_extract,
#search_filters a,
.menu,
.menu #menu_items_scroller{
  background: var(--background-elevated) !important;
}

pre,
ts-message .action_hover_container .btn_msg_action,
.file_container .CodeMirror .CodeMirror-code>div pre, .file_container .CodeMirror .sssh-line pre, .file_container .sssh-code .CodeMirror-code>div pre, .file_container .sssh-code .sssh-line pre,
.search_message_result .search_message_result_meta a,
.search_message_result .search_message_result_meta .date_links a,
.flexpane_redesign #flex_contents .heading_text,
#flex_contents .subheading,
#search_filters.files #filter_files, #search_filters.messages #filter_messages,
.menu ul li a{
  color: var(--text) !important;
  border-color: var(--border-dim) !important;
}

.action_hover_container {
  border: 1px solid var(--border-dim) !important;
}
.ts_tip_float.btn_msg_action:not(:last-child) {
  border-right: 1px solid var(--border-dim) !important;
}
.ts_tip_float.btn_msg_action:hover {
  background-color: var(--background) !important;
}

#quick_switcher_btn{
  /*border-top-color: #333 !important;*/
  background-color: transparent;

}

.client_container #col_channels_footer {
   background-color: rgba(0, 0, 0, 0.5) !important;
   margin-top: 2px;
}

.rxn,
.search_result_with_extract,
.flexbox_client.flexpane_redesign.flex_pane_showing #col_flex,
.flexpane_redesign #flex_contents .heading,
#flex_contents .subheading,
.search_segmented_control,
.menu{
 border-color: var(--border-dim) !important;
}

.search_result_with_extract {
  box-shadow: 0 1px 10px #555 !important;
}

.comment .mention, .feature_name_tagging_client .comment .mention, .feature_name_tagging_client ts-message .mention, ts-message .mention,
span.match {
  border: 1px solid var(--border-bright);
  background-color: var(--background-elevated) !important;
  padding: 2px 4px;
  margin: 2px;
  border-radius: 10px;
}

pre.special_formatting{
  background: var(--background-elevated) !important;
}

code{
  color: var(--text-alt) !important;
  background-color: var(--background-elevated) !important;
  border-color: var(--border-dim) !important;
  font-size: inherit !important;
  font-family: inherit !important;
}

.supports_custom_scrollbar:not(.slim_scrollbar) #archive_msgs_scroller_div::-webkit-scrollbar-thumb,
.supports_custom_scrollbar:not(.slim_scrollbar) #msgs_scroller_div::-webkit-scrollbar-thumb,
.supports_custom_scrollbar:not(.slim_scrollbar) #col_channels:hover #channels_scroller::-webkit-scrollbar-thumb {
   /* scrollbar draggy bit */
  color: transparent !important;
  border: 1px solid var(--scrollbar-border) !important;
  background-color: transparent !important;
}
.supports_custom_scrollbar:not(.slim_scrollbar) #archive_msgs_scroller_div::-webkit-scrollbar-track,
.supports_custom_scrollbar:not(.slim_scrollbar) #msgs_scroller_div::-webkit-scrollbar-track,
.supports_custom_scrollbar:not(.slim_scrollbar) #col_channels:hover #channels_scroller::-webkit-scrollbar-track {
   /* scrollbar endcaps */
  color: transparent !important;
  border: 1px solid var(--border-dim) !important;
  background-color: transparent !important;
}

a, a:link, a:visited{
  color: var(--text-special) !important;
}
ts-message a .mention{
  color: var(--text-special) !important;
}
a:hover{
  color: var(--text-special) !important;
}
/*.channels_list_holder ul li.mention a, .channels_list_holder ul li.unread:not(.muted_channel) .channel_name, .channels_list_holder ul li.unread:not(.muted_channel) a.group_name, .channels_list_holder ul li.unread:not(.muted_channel) .im_name, .channels_list_holder ul li.unread:not(.muted_channel) .mpim_name, .channels_list_holder ul li.unread:not(.muted_channel)>a:link{
  color: #fff !important;
}*/
.channels_list_holder ul li a.channel_name, .channels_list_holder ul li:not(.unread) .group_name, .channels_list_holder ul li .im_name, .channels_list_holder ul li .mpim_name, .channels_list_holder ul li>a:link,
ts-message a.timestamp{
  color: #aaa !important;
}
#channels_scroller.show_which_channel_is_active ul li.active .im_name{
  color: #fff !important;
}

#msgs_div .unread_divider.no_unreads .divider_label{
 background: #000 !important;
 color: #aaa !important;
}

#msgs_div .unread_divider.no_unreads hr{
  border-top-color: #888 !important;
}

.flexpane_redesign #member_mentions {
  background-color: var(--background);
}

/* files */
.file_container.generic_container .file_header_meta .meta_hover {
   background-color: transparent !important;
}

.file_preview_action.btn.ts_tip, .file_preview_link.btn.file_comment_link {
   border: 1px solid var(--border-bright) !important;
   background-color: var(--background-hover) !important;
   color: var(--text) !important;
}
.file_preview_action.btn.ts_tip:hover, .file_preview_link.btn.file_comment_link:hover {
   background-color: var(--background) !important;
}

/* other */
.special_formatting_quote .quote_bar {
   background: var(--border-bright) !important;
}

#channel_scroll_down, #channel_scroll_up {
   background-color: var(--background) !important;
   border: 0 1px 1px 1px solid var(--border-bright) !important;
}

#team_menu, #quick_switcher_btn, #team_menu_overlay, #col_channels_overlay {
   border-color: var(--border-bright) !important;
}