Ves a Panel Control -> General -> CSS personalizado y añade la siguiente línea:
@import url(https://prayag17.github.io/JellyfinCSS/JellyfinCUSTOM.css);
mi Fork
@import url(https://ugeek.github.io/Jellyfin/JellyfinCSS/JellyfinCUSTOM.css);
@import url('https://ctalvio.github.io/Monochromic/default_style.css');
mi Fork
@import url('https://ugeek.github.io/Jellyfin/Monochromic/default_style.css');