Get span from buttons and use it with my custom css

How To
1

Hi all!!! how i can get the span from the bricks button and use it with my custom css?
Like now i can change just the properties of the button in my css but not the span of the button.

Like this:
root:hover span { color:#ff0000; z-index:20;}
or
root .bricks-button:hover span { color:#ff0000; z-index:20;}
or???

Thanks
Davide