Add `color:white` in night mode.

One way to fix [issue #1440](https://code.google.com/p/ankidroid/issues/detail?id=1440).
This seems to work. That is, when there is a color set in the card style or when night mode is off, nothing is changed in the display, and when there is no color set you get white-on-black in night mode, instead of black-on-black.
But i am not sure that this is the RIght Way. Maybe the `ForegroundColor` of the webview should be set, or something. I don’t really like that this only works because of the order the different styles, nor that it sets a style for `html`, which seems a bit odd, but is the only way i can see to make this work when the user sets a color for `html` or `body`.