Flat Light Theme: Build progress and some icons are barely visible (readable)

This applies to latest 4.1 beta and also to the build of PR https://codereview.qt-project.org/165623.

Currently some parts of the interface, eg. icons are suitable for Dark Flat, but in light theme, we get gray on gray elements almost not readable.

1. Build progress bar is barely readable, gray on gray, can we give it a color? Eg. blue, or alternatively increasing contrast there.

2. Some yellow icons on gray background are barely visible: eg. warning icons or navigation icons. See attached screenshots.