karla,
Since OutSystems Platform generates the html pages based on the model you define visually in Service Studio, the identifiers of the html elements is also generated. There's a runtime property Id that allows you to refer to this Id in the application logic but you can't use it for CSS, because CSS is static. Therefore the way to use CSS in OutSystems is to use rules based on classes or html tags and not specific elements. Additionally you can use the extended properties to apply styles to the element. These would be applied in the style attribute of the element.
Since OutSystems Platform generates the html pages based on the model you define visually in Service Studio, the identifiers of the html elements is also generated. There's a runtime property Id that allows you to refer to this Id in the application logic but you can't use it for CSS, because CSS is static. Therefore the way to use CSS in OutSystems is to use rules based on classes or html tags and not specific elements. Additionally you can use the extended properties to apply styles to the element. These would be applied in the style attribute of the element.