You can do that by using roles.
- Create a new Role by clicking assets in the app browser and selecting “Configure roles…” in the drop down. Call it, for example, “myborder”. For now don’t restrict it to a type.
- In “theme.css” add a CSS class called “anvil-role-myborder”
- Now, in the properties section for your Label control you will see a role drop down with “myborder” as an option. Selecting this will add the CSS to that control.
Does that help?