Unfortunately there’s no book or guides that I’m aware of on RapidWeaver 8. There is a limited manual on RW7, and there were a couple of books on older versions but most are way out of date.
About the best you can do right now is the various videos and ask questions here on the forum.
I don’t know what videos you have watched on the themes, but Ben has one here on the new themes with RW8. The first thing he does is change the banner background color and image on the banner for artful.
Have a look and if you’re still having questions about it ask here again and We’ll help you out.