slack.css

.light_theme .message {
  color: #fff;
  font-size: 16px;
}
 
.message .mention {
  font-weight: bold;
}
 
.member_preview_link,
.channel-list-more,
#channel-list .channel,
#im-list .member,
.channels_list_holder h2 ,
#groups #groups_header,
.list_more,
#group-list .group {
    display: none;
    
}

#channel-list .channel.unread,
#channel-list .channel.active,
#im-list .member.unread,
#im-list .member.active,
#group-list .group.unread,
#group-list .group.active {
    display: block;
}
#channel-list .channel.mention a {
    color: yellow !important;
}