#2 Set alert background and foreground from theme

[Timothy Bourke]

Currently the alert.bg.color and alert.fg.color properties set the color of buttons in an alert.

This patch adds two new properties, alert.button.bg.color and alert.button.fg.color, to set the button colors, and makes the original properties affect background and foreground colors of the alert itself.

Without this patch alerts generated by the FireScreen.showAlert() method are transparent.

[Timothy Bourke]