Generally, if you want some HTML to render in a prettier way, you do it by adding CSS.

For instance, if you generate the HTML like this:

import difflib
import sys

fromfile = "xxx"
tofile = "zzz"
fromlines = open(fromfile, 'U').readlines()
tolines = open(tofile, 'U').readlines()

diff = difflib.HtmlDiff().make_file(fromlines,tolines,fromfile,tofile)

sys.stdout.writelines(diff)

then you get green backgrounds on added lines, yellow on changed lines and red on deleted. If I were doing this I would take take the generated HTML, extract the body, and prefix it with my own handwritten block of HTML with lots of CSS to make it look good. I'd also probably strip out the legend table and move it to the top or put it in a div so that CSS can do that.

Actually, I would give serious consideration to just fixing up the difflib module (which is written in python) to generate better HTML and contribute it back to the project. If you have a CSS expert to help you or are one yourself, please consider doing this.