Use light (white) up button for dark themes

This replaces the black button with a white one when using the activity in an application with a dark background:
![screen](https://f.cloud.github.com/assets/813206/732263/1cd5a982-e27f-11e2-96c7-9cff931e2a47.png)


This is done by querying the current theme's background color, converting it to grayscale and checking if it is a "dark color" (value < 128).