Customized Displays: OK to share?

I posted this question in support - but I suspect my answer lies in this thread: Display options for an external wide display (1920x480) - Customised Web Output or Roon Application

I am trying to alter the Web Display for a 1920x480 monitor.

However, I started to fiddle with the Display_ui.html to just get a feel for it, but it doesnt appear to do anything?

I am running Roon Server and editing the file stored here. It has no effect at all. To check, I did a quick edit on the colocated index.html file and changed the title, and this worked - which makes me think it is using the server etc.

C:\Users\MYCOMPUTERNAME\AppData\Local\RoonServer\Application\200001311\webroot

Any ideas why changing this does nothing? Is it possible cached or not referring to the CSS file? I have even deleted it, and it has no effect: