Fix Imperium: Greek Wars (1183470) launcher scrollbar largely drawn as transparent.

The application sets a hardcoded #ffffff as the color key for transparency. On Windows, the
scrollbar parts only have very few #ffffff pixels so the scrollbar is drawn as basically opaque.
If a theme that have many 0xffffff pixels is being used, then the bug reproduces as well on Windows.
I don't want to change the light theme style for this. So let's work around this by using #fefefe
instead of #ffffff. This have little difference visually.

Based on investigation done by Santino Mazza.