slack-theme.css

ts-message:hover:not(.standalone):not(.multi_delete_mode):not(.highlight) {
    background: #353535;
}

ts-message {
    font-size: .8375rem !important;
}

#col_channels,
#team_menu,
#quick_switcher_btn {
    background: #2B2B2B !important;
}

.channel_title_info,
.channel-header .flex_header,
.flex_header,
#channel_header_info,
#primary_file_button,
#col_messages > .row-fluid,
#client-ui,
.client-ui,
.channel_header_refresh #client_body:not(.onboarding):before,
#footer,
#messages_container,
.day_divider,
.tab_container,
.panel .heading,
#mentions_options
 {
    background: #000 !important;
}

.ts_icon:not(.ts_icon_presence):before, ts-icon:before,
.panel .heading,
.message_body,
.timestamp,
.initial_comment .comment,
#im_title,
#channel_title,
#mentions_options {
    color: #fff !important;
}

#quick_switcher_btn {
    border-top: none !important;
}

.channel_header_refresh #client_body:not(.onboarding):before {
    border-bottom: 1px solid #4C4C4C;
}

.monkey_scroll_bar,
.monkey_scroll_handle_inner {
    background: #333131 !important;
}

#stars_toggle.active,
#recent_mentions_toggle.active {
    background: #6D6D6D !important;
}

.action_hover_container .ts_icon:before,
.action_hover_container .ts_icon:after,
#menu_items .ts_icon:before,
#menu_items .ts_icon:after {
   color: #000 !important;
}

#search_container input,
#message-input {
    background: #000 !important;
    color: #fff !important;
}

.special_formatting {
    background-color: inherit !important;
    color: #39cc2f !important;
}

.mention {
    background: inherit !important;
    color: #e1dc28;
}

i think it's time to update the theme. Slack updated their version and it no longer works