Learn to
reverse engineer your book.
Sigil is a fantastic tool for doing this.
Remember, EPUB uses the Box Model and inheritance.
So you start at the span/Paragraph and work outward through towards the Page level until you find what affects what you want (and does not mess the rest
)
Sometimes, you tweak a value, other times, yau specify attributes that were not present (or available to be inherited).
My choice is to set the overall style at the Body level, and make finer adjustments from there for individual elements.
In other words, I only set EXCEPTIONS to the basic overall style.