Guest Book page is large and in all caps

I can only give you an example for some of the biggest style issues:
#comments { font-size: 16px; text-transform: none; }

You have to add this code in one of your css-files.

Thanks, but when I add that code to the place designated for custom CSS in my theme, I don’t see any change. The page where I have the plugin is the only page where I have this font issue.

Ok, please try this:
#comments { font-size: 16px !important; text-transform: none !important; }