Fixes: Set alternate base on light themes

Details:
Not all themes have ok contrast for alternate base and
we want to keep it light, so I'm enforcing it for
themes with white background.

src/plugins/projectexplorer/projecttreewidget.cpp

@@ -48,6 +48,7 @@
 #include <QtGui/QVBoxLayout>
 #include <QtGui/QToolButton>
 #include <QtGui/QFocusEvent>
+#include <QtGui/QPalette>
 
 using namespace ProjectExplorer;
 using namespace ProjectExplorer::Internal;
@@ -74,6 +75,11 @@ public:
         setUniformRowHeights(true);
         setTextElideMode(Qt::ElideNone);
         setAlternatingRowColors(true);
+        QPalette pal = palette();
+        if (pal.base().color() == Qt::white) { // Leave dark themes as they are
+            pal.setBrush(QPalette::AlternateBase, QColor(239, 239, 239));
+            setPalette(pal);
+        }
         setProperty("AlternateEmpty", true); // Let Manhattan to override style default
 //        setExpandsOnDoubleClick(false);
     }