There are several outstanding points in this article. What you call "theme", I've always called "premise" (as described in The Art of Dramatic Writing by Lajos Ergi, which is about writing plays, but the principles often apply to prose too, especially concerning premise). Also, themes (or premises) are as vital for genre fiction as so-called "literary" fiction, if you ask me. For example, I writer horror-thriller mysteries. My most recent mystery thriller novel Death Nest is about the need to overcome false guilt to come to terms with personal tragedy.

I would add that as a full-blown plotter, I only ever write a novel if I first have an ending that absolutely blows me away, proving my premise (or theme). I then work backwards from that point with character outlines, plot outlines, other research, etc. A lot of "pantsers" consider this rigid and inflexible, but it really isn't. More on my process here, if you're interested. :)

https://writingcooperative.com/the-big-myth-about-plotters-bd565a99ecba