So an example of instructions on https://www.domoticz.com/wiki/Machinon_Theme =
cd domoticz/www/styles
git clone https://github.com/EdddieN/machinon-domoticz_theme.git machinon
sudo /etc/init.d/domoticz.sh restart

to change themes after connecting to root of Synology:
cd /usr/local/domoticz ...