You’ve specifically asked me for advice on this in another thread, but I’m no ROCK user myself, so can’t tell you where to locate the file…
I’m using Windows on my core, run Roon server and additionally full Roon if I want to take control from there - of course, I have to change display_ui in the //…/roonserver, not //…/roon directory for the changes to effect any web display.
Sorry to be of no real help, but maybe some ROCK users can chime in here!
Thanks for trying to help!! It is really much appreciated. As you said, maybe a ROCK user can help me get the display variant you designed to work.
Unfortunately the Roon Rock (Linux) Core has hidden (and encrypted) system files. Same with my Nucleus. No way to change the display_ui. And even if that was possible - every update would overwrite the individual changes made.
It works for the Roon Core running on Windows … but WIN is not, what Roon OS was developed for.
Have a look at my thread from last year, some visualisations (and Comments from Roon 
) (User selectable) Alternative Designs for Web-Display