Roon Web Display
Since Roon Build 354 it’s possible to display Now Playing information and lyrics in your webbrowser and Chromecast devices.
FireTV devices
It’s possible to display it on FireTV devices with Firefox and Silk.
To prevent the screen from shutting off after a while, I use the app Fully Kiosk Browser on my Android tablet and FireTV (there are different apks available).
My Modified Version (obsolete)
Since (Roon Build 923), the web display has been heavily revised. I am satisfied with this solution on the whole, so I see no need for me to manually adjust the corresponding files anymore.
With Roon-Server installed you can modify the file /RoonServer/Appliance/webroot/display_ui.html.
If you use ROCK it is not possible to modify this file.
I have used the browser extension Stylus where I can put this CSS code into without fiddling around with Roon server files, which are overwritten again when Roon is updated. The use of Stylus cannot be applied when viewing with Chromecast.