Change default color to neutral grey

The override for this is really nasty due to how tightly scoped the default styles are. This is why I previously had some color options in the plugin initialization. Although I definitely understand the decision to take them out, it might be worth thinking about a way that we can make style overrides easier.

Maybe something like including an additional `override` class on the element that we don't use in any of the internal styles, but if used externally would win the specificity battle?

Closes #108