slack-theme.css

 body, .channel_header, # footer, .channel_title_info, # channel_topic_text {
     background: rgb(0, 43, 54);
}
 .c - message__body {
     color: rgb(153, 174, 177);
}
 # team_menu, .p - channel_sidebar {
     background: #023f4e !important;
}
 .c-presence--active {
    color: rgb(177, 202, 17) !important;
}
 nav.p-channel_sidebar .p-channel_sidebar__channel--selected, .p-channel_sidebar__link--selected, .c-message_list__day_divider__label__pill, .p-message_pane .c-message_list.c-virtual_list--scrollbar > .c-scrollbar__hider:before {
     color: # eee!important;
    background: rgb(27, 139, 210) !important;
}
 .c - message_list__day_divider__line {
     border - top - color: rgb(27, 139, 210) !important 
}
 # msg_input, # primary_file_button {
     background: rgb(2, 55, 68) !important;
}
 # msg_form # msg_input {
     border - color: transparent;
}

to apply the above theme to slack use:
https://gist.github.com/ruggertech/5427683eb911954e38e10e66c715d51c