[themes] Allow customization of palette role colors

This fixes the following issues:
- combo box drop down menu white top/bottom edges
- color button header background color
- white line at top of stacked dock area

src/core/qgsapplication.cpp

@@ -36,6 +36,7 @@
 #include "qgsrasterrendererregistry.h"
 #include "qgsrendererregistry.h"
 #include "qgssymbollayerregistry.h"
+#include "qgssymbollayerutils.h"
 #include "qgspluginlayerregistry.h"
 #include "qgsmessagelog.h"
 #include "qgsannotationregistry.h"
@@ -759,6 +760,28 @@ void QgsApplication::setUITheme( const QString &themeName )
   file.close();
 
   qApp->setStyleSheet( styledata );
+
+  QFile palettefile( path + "/palette.txt" );
+  QFileInfo paletteInfo( palettefile );
+  if ( paletteInfo.exists() && palettefile.open( QIODevice::ReadOnly ) )
+  {
+    QPalette pal = qApp->palette();
+    QTextStream in( &palettefile );
+    while ( !in.atEnd() )
+    {
+      QString line = in.readLine();
+      QStringList parts = line.split( ':' );
+      if ( parts.count() == 2 )
+      {
+        int role = parts.at( 0 ).trimmed().toInt();
+        QColor color = QgsSymbolLayerUtils::decodeColor( parts.at( 1 ).trimmed() );
+        pal.setColor( static_cast< QPalette::ColorRole >( role ), color );
+      }
+    }
+    palettefile.close();
+    qApp->setPalette( pal );
+  }
+
   setThemeName( themeName );
 }