Creating a fixed header in Arjuna-x

I’m having issues implementing suggestions about creating a fixed header (page scrolls as if going under header) in OTHER themes. However, the updated coding in Arjuna-x, a theme I am using, seems a bit different.

What is the code, and how do I implement it, to create this fixed header in Arjuna-x? The theme, I believe, is capable of this change….just scratching my head as to where to put the code.

Thanks.