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nor change the css file in each book individually
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This is really the stumbling block. Your desire for bold text everywhere is unusual enough that there is no automagic fix, at least that I've ever seen. Epub books totally rely on the css.
You will find that in most books, the font-weight lines are simply missing from most text styles--the assumption of "normal" is everywhere. So you would have to either add font-weight: bold everywhere needed, or embed a bold font.
You might find you can add it to some conversion setting, so you could just do a conversion and have it done, but I can't tell you exactly how since I don't use conversion for that sort of thing. Embed a dark font during conversion?