Paste inside Appearance -> Custom CSS
Make sure Dark Mode is enabled.

@media (min-width: 768px) {
body {
    background-image: url('http://i.imgur.com/KkC2kU3.png');
    background-repeat: repeat;
  }

   #flarum-loading {
   

    font-size: 24px;
  }
  #header {
    background-color: rgba(14, 14, 14, 0.7);
    height: 65px;
    max-width: 1100px; margin: auto auto;
    margin-top: 10px;
    border-top-left-radius: 4px;
    border-top-right-radius: 4px;
  }
  .Hero {
    margin-top: 10px;
    border-top-left-radius: 4px;
    border-top-right-radius: 4px;
  }
  .App-content {
    border-top: none;
    background-color: rgba(14, 14, 14, 0.7);
    max-width: 1100px;
    margin: 40px auto;
    border-radius: 4px;
  }
  .App:before {
    display: none;
  }
  .hasPane.panePinned .App-content, .hasPane.panePinned .App-footer {
    margin-left: 30%;
  }
  .UserCard {
    border-top-left-radius: 4px;
    border-top-right-radius: 4px;
    overflow: hidden;
  }
  .containerNarrow {
   

  }
  .DiscussionHero-title {
   

  }
  .Button-label .username {
    

    font-size: 16px;
  }
  .TagLabel {
    font-size: 100%;
    font-weight: normal;
  }
  .TagCloud {
    padding-bottom: 20px;
  }
  .sideNav .Dropdown--select .Dropdown-menu > li > a {
    padding: 8px 0 10px 65px;
  }
  .App-header .container {
    padding-top: 12px;
  }
  .App-header .Header-navigation {
    margin-top: 10px;
  }
  .Scrubber-handle {
    background: none;
  }
  @media (max-height: 820px) {
    .App {
        padding-bottom: 0 !important;
    }
  }
  .DiscussionPage-discussion > .container {
    min-height: 520px;
    padding-bottom: 20px;
  }
  .DiscussionPage-list {
    top: 75px;
    width: 370px;
    border-top: none;
    background-color: rgba(14, 14, 14, 0.7);
  }
  .ReplyPlaceholder {
    margin-left: 5px;
    margin-top: 25px;
    padding-left: 150px;
    font-size: 18px;
    

  }
  .Avatar {
    

  }
  #admin-navigation {
    top: 85px;
    border-radius: 4px;
  }
}
.PostsUserPage-discussion a {
  font-style: normal;
}

Preview:

Modified from: https://github.com/jsthon/flarum-custom-css-style

Todo:
Optimize for retina displays.