Use less specific CSS selectors for the default font.

Now that we've fixed the CSS selectors for the default font, a new problem with their specificity has emerged in [Issue 1739](https://code.google.com/p/ankidroid/issues/detail?id=1739). Since .question and .answer appear inside .card, we are overriding any font specified by users using .card (a common case).

This changes the selector to simply target the body element.