Address review comments

src/citra_qt/configuration/configure_graphics.cpp

@@ -109,7 +109,7 @@ void ConfigureGraphics::updateShaders(bool anaglyph) {
     }
 }
 
-void ConfigureGraphics::UpdateBackgroundColorButton(QColor color) {
+void ConfigureGraphics::UpdateBackgroundColorButton(const QColor& color) {
     bg_color = color;
 
     QPixmap pixmap(ui->bg_button->size());