Fix disabled arrow styling on Default theme

resources/themes.qrc

@@ -1,6 +1,7 @@
 <RCC>
     <qresource prefix="/">
         <file>themes/dark.qss</file>
+        <file>themes/default.qss</file>
         <file>themes/midnight.qss</file>
     </qresource>
 </RCC>

resources/themes/default.qss

@@ -0,0 +1,25 @@
+/* DEFAULT theme for porymap */
+
+/*  
+    For newer versions of macOS at least, icons
+    using Qt::ArrowType do not change color when
+    disabled, so we specify the color change ourselves.
+*/
+QToolButton {
+    color: black;
+}
+QToolButton:disabled {
+    color: gray;
+}
+QComboBox {
+    color: black;
+}
+QComboBox:disabled {
+    color: gray;
+}
+QAbstractSpinBox {
+    color: black;
+}
+QAbstractSpinBox:disabled {
+    color: gray;
+}

src/mainwindow.cpp

@@ -669,14 +669,10 @@ void MainWindow::restoreWindowState() {
 }
 
 void MainWindow::setTheme(QString theme) {
-    if (theme == "default") {
-        setStyleSheet("");
-    } else {
-        QFile File(QString(":/themes/%1.qss").arg(theme));
-        File.open(QFile::ReadOnly);
-        QString stylesheet = QLatin1String(File.readAll());
-        setStyleSheet(stylesheet);
-    }
+    QFile File(QString(":/themes/%1.qss").arg(theme));
+    File.open(QFile::ReadOnly);
+    QString stylesheet = QLatin1String(File.readAll());
+    setStyleSheet(stylesheet);
 }
 
 bool MainWindow::openProject(QString dir, bool initial) {

src/ui/preferenceeditor.cpp

@@ -38,7 +38,9 @@ void PreferenceEditor::initFields() {
     QDirIterator it(":/themes", QDirIterator::Subdirectories);
     while (it.hasNext()) {
         QString themeName = re.match(it.next()).captured(1);
-        themes.append(themeName);
+        if (!themes.contains(themeName)) {
+            themes.append(themeName);
+        }
     }
     themeSelector->addItems(themes);
 }