Currently it is not possible to bulk update answer format (font, color, size etc.) in projects. We can set a format only if we are doing bulk answer using the 'Answer questions' option - which we use to add the same answer for all the selected questions (screenshot below).
But there should be an option to bulk update the format of already existing answers - the same way we can do it content library using the 'Format Content' option.
Here is a reference ticket -
https://responsiveio.zendesk.com/agent/tickets/64210