The stuff you would mess with
is fairly simple, done with a text editor (not a Word Processor which will make a mess) like Notepad.exe (notepad++ is a free replacement that colorizes the scree to match parenthsis/braces and keywords for ease of getting thing paired up.
Make a copy of an EPUB file.
Unzip it into a temporary folder (You can add .zip to the filename or "Open With" <your unzip program>
Look for the css file(s).
Pick the first one (if numbered), Edit
A lot of the things will make sense, like Margins
What you need to figure out, is what way the original book designer used to specify things.
pt, em, % (of screen space)
Now that you have your head wrapped around this stuff, Time to figure out which one of thos do you change.
To start, open the book section you want to change in
another text editor window So you can flip between the css and the page.
What you are looking for is the controlling style tag in the book
Now find the EXACT same name in the css (beware of similar: txt, txt1)
I wanted my font to default a little bigger (without resorting to the PEz zoom).
so I found the Body tag and changed the font from .978em to 1.1em
Oh and I wanted
left justify, not
justify, so I changed that line also
All the files up, and change the name back to .EPUB and tryit on a reader/program.
Sigil .2 takes care of all the work, just open the EPUB with Sigil
Find the Styles section.. doubleclick..edit
find the chapter in Text... doubleclick and see