Nice find. Unfortunately, as you predicted, Flarum's styling has moved on and their custom CSS no longer works as written.

a month later

I made pretty similar styling. Didn't know for this one.
Demo: https://etforum.freeflarum.com/
So here is code:

body::-webkit-scrollbar {
    width: 0.3em;
}
 
body::-webkit-scrollbar-track {
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
    background-color: white;
}
 
body::-webkit-scrollbar-thumb {
  background-color: purple;
  outline: 1px solid slategrey;
}
body{
background: url("http://www.technocrazed.com/wp-content/uploads/2015/12/HD-purple-wallpaper-image-to-use-as-background-14.jpg");
background-size:cover;
background-attachment:fixed;
background-position: center center;
background-repeat:no-repeat;

}
.Hero{
background-color: rgba(255, 255, 255, 0.35);
border-radius: 5px;
}

h3, a{
color:black !important;
}
.App-content {
background-color: rgba(255, 255, 255, 0.8);
max-width: 1200px;
margin: 20px auto;
border-radius: 5px;
  }
 .DiscussionPage-discussion > .container  {
    min-height: 530px;
  }
.App:before{
background-color: rgba(255, 255, 255, 0.9);
}
.UserPage{
border-radius: 5px;
}
.PostStream-item{
border-bottom:1px solid #e5aaff !important;
}
.DiscussionListItem:hover{
background-color:#e0beff;
}
.Scrubber-handle{
background:none;
}

For color themes i used: highlight color : RGB(77, 0, 96), and for second color: RGB(106, 10, 148)
I hope this will be helpful for someone ?.

    Razor That is awesome! Mind if I include you in the FreeFlarum showcase gallery?

        Razor I made pretty similar styling. Didn't know for this one.
        Demo: https://etforum.freeflarum.com/

        One suggestion:

        .Hero {
    background-color: rgba(255,255,255,0.35);
    border-radius: 5px;
}

        This way, you preserve a little more background shining through. Also it's less greyish.

          Thank you for suggestion. I appreciate that. I wanted hero to be separated from rest, so i added a grey tint, but it actually looks better with changes u said. So i will include that. If someone want to customize style, but do not know how, i will help (changing colors etc). I will be making few more styles, and will post them when they are done.
          I want to ask Sanguine if it's ok to create a one, or two more forums using freeflarum, for testing purposes?

              Razor absolutely, go ahead! Eventually it would be nice to have a theme gallery. So many wishes for the future ?

                Razor The background is too outdated. The High Sierra one should be better.

                    Meow
                    You are probably right, but i didn't put this bg because of mac os theme or whatever, but because it was the bg that was most fitting in my mind for this style. But, thank you for suggestion.

                      a year later
                      9 months later