Hack font size down to 11pt on Mac.

Fixes issue #546

data/mainwindow.css

@@ -1,3 +1,7 @@
+darwin {
+  font-size: 11pt;
+}
+
 #playlist {
   background-color: %palette-base;
   background-image: url(:logo.png);

src/core/stylesheetloader.cpp

@@ -73,6 +73,10 @@ void StyleSheetLoader::UpdateStyleSheet(QWidget *widget) {
   ReplaceColor(&contents, "Link", p, QPalette::Link);
   ReplaceColor(&contents, "LinkVisited", p, QPalette::LinkVisited);
 
+#ifdef Q_OS_DARWIN
+  contents.replace("darwin", "*");
+#endif
+
   widget->setStyleSheet(contents);
 }