Which JMP version are you using? Starting with 1.11.0 of JMP, by default it uses the server's jellyfin-web for rendering. So if you have CSS that works in a browser it should also carry over to JMP.
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)