I would not go so far as use plain text, at least in the beginning. Just do a search for references to font size and face and eliminate them and save it with a new file name and see if you like the result. Just keep changing, always working with a copy, until you are happy with the result.
You can check out this website: http://www.w3schools.com/css/
to learn about CSS and html in general to get an idea of how things work.