What gives out that it is a she and not a he?
I think because ráda is for a woman and rád is for a man
She - ráda. He - rád. They - rády, rádi.
Why is ma used? Instead oni, or just he she it version of the verd radost
Má is the third person singular of mít, the full expression would be Ona má, because ráda is feminine.
ráda can also be plural neuter, so one could also use Mají ráda in some other context for "they like bread".
Why not 'she prefers white bread'?
That is comparative, If you prefer something, you like it more than something else. "Má raději bílý chléb." "Preferuje bílý chléb." "Dává přednost bílému chlebu." "Upřednostňuje bílý chléb."
Thank you once again for your quick answer. Tchech is very hard to learn.