The CSS from Standard EBooks is rubbish. They have no idea how to do is properly. KISS is the way to do it. They just want to use the most convoluted CSS they can. They even have some code in their CSS that causes RMSDK on Kobo to ignore the entire CSS. It was pointed out to them what the problem way, but they don't care.
I did take one of their book and instead of always trying to figure out what the CSS was doing, I just made simple CSS to do what needed to be done based on looking at the way the eBook displayed in Calibre's viewer.
Have a look at
What exactly are Standard Ebook's "advanced epubs"? and when you get to post #34, download the eBook I posted and comoare the CSS from mine to theirs and you'll understand the problem.