On a technical level I don't think there are major problems with your "hack", though it is quite inelegant.
But...
I personally only go in to change specific items that annoy me (like too small line spacing, too small font size for regular text, mega-large margins etc) because I want to preserve as much of the existing formatting as possible. I want to believe that the "type-setting" of each book had some thought put into it to match the subject and mood etc. I don't want the responsibility of being the one who decides that, and I don't want ever book I read to look the same.
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