Headline Widget’s mobile CSS Style, how to write?

Hi, I’ve complete 1 page design for my client. The page looks good in desktop but when view in mobile, the font showed by Headline Widget is way too big. I see under Attribute, there is a mobile CSS Style, but I do not know how to code. How do I define the H1 tag (from particular Headline Widget) to have smaller font size when in mobile version? Hope you can help.