Thanks @Canapin 
I fixed the code typos.
Categories and Sub-Categories can also be used in CSS by using category-
i.e…
.category-books .main-content,
.category-my-great-book .main-content {
width:100% !important;
}
I’m also using this trick on my site extensively, to style content and embeds on-the-fly inside the composer…
I have a section of my site where I document my developments (mainly so my future self can recall and find what I have already done, so I don’t continue to reinvent my own wheels!) But anyone is welcome to visit and/or subscribe to that category if you’d like…