Step 1: Import to your CSS with
@import url("https://cdn.jsdelivr.net/gh/tedhinklater/finimalism@main/finimalism7.css");
or if you don't want all the lovely colours...
@import url("https://cdn.jsdelivr.net/gh/tedhinklater/finimalism@main/finimalism-just-black.css");
Step 2: In Jellyfin Client / Web UI Settings, under "Display", the Theme is set to Dark and Backdrops are enabled.
Everything is just an alteration of the amazing work already put into Ultrachromic by CTalvio. Updated for jellyfin server 10.9.4