From is a span class of from so .from should target it. It doesn't seem to and I'm not sure why. Perhaps contact the makers of the theme. Obviously something in their code is preventing that change.
I checked a firebug of the Mystile theme and it is a class of from but when I altered it, it did not change. Maybe try a google search for that specific theme. It's good to be putting your new css in the custom.css by the way, like you're doing. Good luck.