Is there any way to get rid of the blue rectangular background for the recent activity section of the Home page? Probably my main gripe with the aesthetics of the interface (except maybe the gigantic ‘Hi, …’ at the top 
) To my eyes it looks awful.
Not easily, you could try and edit the colors file, but, it is manual and you might have to redo it every update change, as well as, risk Roon GUI breaking. Look in the Tinkering section.