There's a lot of concern around full site editing and what that means for themes and theme creators. Are they even worth building anymore? Should all themes just be a blank slate? Will we only need a hand full of themes now?
NO! In-fact, good themes will be more important than ever now. Given the amount of flexibility the block editor and full site editing offer users, have well-designed themes with all the right guideposts is imperative.
So if you're a theme developer, or want to be one, here's what you should focus on.
Resources: https://casabona.org/fse/