[SpeedDial] Fix overlay text input visibility with dark themes

Reported on [QupZilla blog](http://blog.qupzilla.com/2014/01/qupzilla-160-released.html?showComment=1389217725819#c1185455227885957978).
How does it look like: https://mediacru.sh/cdaa67526767
* Image 1: not changed.
* Image 2: with the changes.
* Image 3: with the changes, dark theme.

Edit: Candidate for v1.6 branch.