Improve standard button theme for dark mode

[dfabulich]

We have a fancy-button class that we use for some buttons on the site, but many buttons are just plain old HTML buttons.

That's fine, but those buttons don't theme themselves in dark mode. But, what color should they be? They need to be prominent enough to notice against a black background. 🤔

Even the fancy-button doesn't have a dark variant. (We didn't inherit any suggestions from MJR's Midnight theme.) It's easy to style now, so we should probably do that, too.