I'd agree with the OP here. Stylus may be "very open and flexible" but I don't know what stylus is either. I came to your page because I wanted to find out about roots, not about stylus.
Open with a statement saying "Roots is a css framework that does X". Maybe have a follow up statement saying "Roots is based on the stylus language".
Open with a statement saying "Roots is a css framework that does X". Maybe have a follow up statement saying "Roots is based on the stylus language".