[stumped]

i think it's harder to automate than you suggest
e.g. styles - does that include bold, italic, italicbold...
justification: I like story text to be justified, but that does to mean I want all headings, poetry verse which may occur in the story to be ruthlessly changed
margins,- for narrative purposes it is often necessary for some margins to be deeper than others
& because CSS is hierarchical you have to consider at what level(s) to apply changes. I get good results( for me ) by having calibre add css only to the main calibre body style - where it can still be specifically over-ridden lower down within the CSS if needed. So I add a top level don't hyphenate ( as I don't like hyphenation) and top level widows 1 orphans 1 as I don't personally care for those effects either.
But I am not tweaking solely for kobo, I often want a book to look good on my kobo reader which my son has on semi-permanent loan, but also to look good in the various android reader apps we use. Mostly that just requires zapping line heights and any forced left justify commands in body text, then let the apps or the device sliders do their things. i only want to have to keep one version per format in calibre
android apps like bookari also add CSS in order to implement their themes, so possibly you end up , in the book, with CSS from publisher, more CSS from calibre plug in , and from the reader app , all asserting their importance while being poked at by the kobo reader sliders... not a pretty sight