I don’t want to open a can of support worms, so as with any files distributed with Roon, make a backup before you edit, expect your changes may be clobbered by automatic Roon updates, don’t bug support when something doesn’t work, and so on. But all of that said, on Windows, the answer is:
C:%homepath%\AppData\Local\Roon\Application\100600416\webroot
My experience is that Roon will see and serve up the changes in realtime, without a restart needed. You just need to reload the Display web page.
The main CSS is inside display_ui.html. You can do almost anything (visually) just by editing display_ui.html. The only things I needed to edit display_ui.js for were to give me a three-line display instead of a two-line display and to change how the waveform canvas is drawn.