Portlet css class

at the bottom of defining a portlet TTW, there is: “Portlet class CSS class to add to the portlet”

What does this mean? More importanly, how is it implemented? I know what a css class is and how to define one. However, don’t know how to implement it here. I want a css for the portlet, so do I add a path here that goes to the css? Or, if i define a class,
.intro {
background-color: yellow;
would i just type in ".intro’ in this line and automagically the class is found (not likely) or type the entire definition of the class?

Finally, does the css have to be part of the theme? or can it be a file somewhere?

Which portlet? And which plone version?

Porlet API version 3.0 tks bro