You might look at my style sheet at
https://notjohnkdp.blogspot.com/2013...yle-sheet.html -- free for the taking; adapt at your pleasure.
It should go in the Sigil "Styles" folder, and note Dion's pointer above. In this case you'd name the file styles.css
Note that I never use an absolute measurement such as points, but rather ems (and maybe percents; I don't remember).