[Schedule View] Light and Dark Colour Scheme Option

[majick777]

While the majority of users use light colored themes, some use dark ones and we get periodic requests for custom CSS to apply light borders etc. so that the schedule views work on a dark background.

To help with this we could provide the option of having a dark theme in the shortcode and built in so the default would be a light theme but provide a dark theme option, similar to how we do for the player button scheme.