The little transition from non-hover to hover is pretty neat, but it doesn't suggest interactivity that much. I propose either making it use cursor: pointer; for the button rule or background-color: /* insert some color or CSS variable here */; for the button:hover rule.
I'd love if there was a dark theme. Could you add one at default-dark.css? I'm configuring my page to load a stylesheet from that URL. I really would like having a dark theme, so please implement it.