Definitivelly not a syntax issue "in the css", it look smore like a pointing issue.
I did copy all second CSS code in the first one and it works.
The style001.css works
If I rename it to style0001.css (all sections pointing to it), it's not working anymore. Like ADE only abble to accept the style001 and nothing else.
What sounds illogic based on your comments, it should work
PS : i renamed the 0001 in 001, and surprise, it doesn't work back, looks like Sigil is messing something to the pointing codes