Unstyled buttons in Safari have background color

Unstyled buttons have a background color, which can be an issue with icon buttons (see https://github.com/dequelabs/cauldron-react/pull/113#issuecomment-495355568). We likely want to normalize these for consistant appearance in all browsers.

![safari button gray](https://user-images.githubusercontent.com/3180185/58281110-2bae9800-7d57-11e9-956f-5b0a2487a524.png)