How can I edit the size/colour of the header text, while leaving the rest of the page unchanged?

Hi,

I want to make the header font, white and smaller in size, while leaving the text on the rest of the page as it is.

How do I do this?

Also, what does the 'assign styles' section do/allow me to do?

Thanks

Design > Site Styles > Fonts > Assign Styles > Site Navigation