yeah, I think (may be wrong) there are some issues with the frontail in openHABian currently. I will try to open up a PR to fix them.
download the required files as follows:
wget "https://raw.githubusercontent.com/openhab/openhabian/master/includes/frontail-theme.css" -O /usr/lib/node_modules/frontail/web/assets/styles/openhab2.css
wget "https://raw.githubusercontent.com/openhab/openhabian/master/includes/frontail-preset.json" -O /usr/lib/node_modules/frontail/preset/openhab2.json
if this doesn’t work, let me know and I can give you some more examples on the service file also