Can't say much of the Stilo's system, but some general advice for previous cars perhaps... After the thermostat is changed, you should flush the system using a garden hose - pretty simple. Then bolt everything up, refill 50/50 coolant/water and let the filler stay open. It might be a drain/air plug somewhere at the bulkhead.
When it is cold it may take ages for the thermostat to open. You will notice this when your coolant starts circulating. After a while - or even never, the fan cuts in to cool off the radiator. When the thermostat opens more coolant will be let into the system - hence a refill will most likely be needed. It was a classic earlier to refill the system after a flush or thermostat change, plug the cap and drive off: Oops low on coolant when the fluid started moving and filling the huge hoses after the thermostat.
This may not relate to the Stilo, but an ok thing to store for later.
M.