You mean the border?
if so: type the following in the style property of the container: border-style: ridge;
border-color:rgb(168,168,168,0.2);
If you want to define it as a project standard for all containers (not only for one container), I would try to do this in the _custom-variables.scss file (local project folder/theme/styles/web/sass/app/_custom-variables.scss) ... or just explore the available css and scss files in all this folders, they are well commented