Well you should be pleased, you are undoubtedly way ahead of what most calibre users would ever do by diving under the covers to modify a book's css styles. Which means you are way more likely to be able to read the books on your reader formatted how *you* want them to look.
There is indeed a lot of voodoo involved with css/html rendering, particularly once you start banging your head against issues to do with styles inheriting and conflicting with each other, but you've made the most important first steps.
Have fun...