Newbie - Custom CSS class?

Good day

I have added the custom html to my home page

I have added "top" to the advances tab Custom CSS class:

Now my question what css file do I add my"top" clas to where I can add custom css.
And where is the css file located?

Thank you

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking