Override WebView font color, for dark themes

The WebView background color is transparent, so if the page itself does
not specify a background color, the text is left black (browser default)
on whatever color is used by the dialog theme.

On Android 2.x, it is dark, and a black text on dark screen does not
often make the best impression. On Android 4/5.x it is light, so a black
text is fine.

The fix consists of reading the dialog theme text color, and updating in
JavaScript (there is no other option) the style of the default text
color, if:
- no text color is already set
- no background color is already set either