MessageView theme toggle: change the theme of the whole activity and not just...

MessageView theme toggle: change the theme of the whole activity and not just the background of the WebView

Using dark theme with white WebView background looks very ugly,
especially when a "download complete message" or a "show images"
button is present.

This change applies the theme to the whole activity.

Also changed the text for the toggle menu. It's also shorter now,
and isn't cut off anymore on hdpi/480px wide devices.