You can just completely blow away the existing CSS. The ereader has somewhat reasonable defaults. Try opening the ebook in calibre's editor and renaming the css files, then save it, then open it in Calibre's viewer. It may not be pretty but it's usable.
(Although the above won't work in the unfortunate situation where there are style= things on the individual html tags. There's a special place in Hades for authors who do that.)
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