Overwriting the font style, color & size

Some of my members are creating Blogs on my site, but they all use different fonts tyle, different size etc.. I’d like to keep the font style & size consistent for all Blogs. I was wondering if there is a way I can modify the code so no matter what font style or font size they choose, it will always get overwritten with my standard font & size I setup behind the scenes..

Thank you.