Perhaps the first surprise when I started fiddling with WordPress 5.9 and a block theme on an old site was that I didn’t see the Customizer at all. For those who’ve not done that, under “Appearance” you might end up with as few as two menu options: “Themes” and “Editor (beta).” So how do you do some Customizer things, like my old favorite of using it to set the Site Icon?
Justin Tadlock covers that whole question in more this article. Part of the answer: the Customizer isn’t gone, just not linked to. So you can just “hack” your way to it at /wp-admin/customize.php. And with that, all is restored.
And I’ll add that this is purely something that happened because I was using the cool Livro theme as opposed to a theme from an earlier era. All older themes will have the Customizer for the foreseeable future, no need for most of us to memorize that URL just yet 
