Hi Diogo,
It's not recommended to make css rules based on id's since you cannot control them at all.
You should created rules based on classes ".yourstylename { .... }" and then set the style property of the widget to "yourstyleme".
It's really weird that you managed to get a css work by id inside Service Studio preview. What did you use?
Regards,João Rosado
I believe the issue is then,
the id is being "generated" by outsystems, so your extended property will not be parsed.
like joao said, just use the class instead of id.