[hobnail]

As theducks says, the only sure-fire way is to edit the code. Unfortunately you need to eyeball the html files to see what classes are used on the body tag as well as the p tag. Then fix the css files.

I take a sledge hammer approach and in the html files I globally delete the class on the body tag as well as the p tag for the body paragraphs (e.g., substitute nothing for class="calibre1"), then I add the following to the bottom of the css files.

body {
    border: 0;
    margin: 0;
    padding: 0;
     width: auto;
}

p {
    margin: 0;
    padding: 0;
    text-indent: 2em;
}

Alternatively, you could fix their css for whatever classes they're using. Also watch out for a div around the entire chapter in each html file, which will also need fixing.

Remember that in the html a p tag with a class has higher precedence over a naked p, regardless of the order in the css file.