I was planning to customize buttons with CSS. But button Styling in Gutenberg is defined by classes that Gutenberg controls. I cannot just add a class to override these. So I can’t define the colors, hover, shadow, etc.
Also, I found that if I set the background color using RGB through the Gutenberg interface, then I lose hover color. BUT, if I choose a system color, this does not happen.
So, my solution is to use a plugin that lets me customize the color pallette. The plugin that I chose is “Block Editor Colors”