change appearance of the “back to the top” arrow

Hello everyone,

I would to know how to change the appearance of the “back to the top” arrow.

I would like to display an arrow in a cirlce, or at least display rounded corners on the existing square.

I also have a second question. I would like to change the font used on the whole site, any simple way to do that ?

Thanks fo your answers ??

The page I need help with: [log in to see the link]