Add support for custom styles

[pfcodes]

Some stores will use widgets that are dependent on the global stylesheet of the store's theme.

It would be useful to allow for a custom global styles.css (nested inside public/assets) to mimic the store's environment.

[christensenep]

Hello @pfcodes, thanks for the suggestion, we will look into doing this. We are also hoping to add ways of allowing widgets to absorb styles from themes without having to access theme css directly, which should make it easier for widgets to play nicely across multiple different themes.