Create master/slave relationship for styles

The dark css is the master and contains all style information. The
light css is the slave and only contains color information.

files/styles/style-dark.css

+/*
+ This is the master style sheet as well as the dark style. This style sheet should contain both
+ color and size/positioning information for all styled controls. This sheet is always loaded first
+ Then the user specified style sheet is loaded after it to override and color settings.
+*/
+
 * {
     background-color: #222;
     color: #FFF;

src/ui/MainWindow.cc

@@ -1067,19 +1067,36 @@ void MainWindow::enableAutoReconnect(bool enabled)
 
 bool MainWindow::loadStyle(QGC_MAINWINDOW_STYLE style, QString cssFile)
 {
+    bool success = true;
+    QString styles;
+    static const char* masterCssFile = ":/files/styles/style-dark.css";
+    
+    // Signal to the user that the app will pause to apply a new stylesheet
+    qApp->setOverrideCursor(Qt::WaitCursor);
+    
     // Store the new style classification.
     currentStyle = style;
     
-    // Load the new stylesheet.
-    QFile styleSheet(cssFile);
+    // The dark style sheet is the master. Any other selected style sheet just overrides
+    // the colors of the master sheet.
+    QFile masterStyleSheet(masterCssFile);
+    if (masterStyleSheet.open(QIODevice::ReadOnly | QIODevice::Text)) {
+        styles = masterStyleSheet.readAll();
+    } else {
+        qDebug() << "Unable to load master style sheet";
+        success = false;
+        goto Error;
+    }
 
-    // Attempt to open the stylesheet.
+    if (cssFile != masterCssFile) {
+        // Load the slave user specified stylesheet.
+        QFile styleSheet(cssFile);
         if (styleSheet.open(QIODevice::ReadOnly | QIODevice::Text))
         {
-        // Signal to the user that the app will pause to apply a new stylesheet
-        qApp->setOverrideCursor(Qt::WaitCursor);
             
-        qApp->setStyleSheet(styleSheet.readAll());
+            styles += styleSheet.readAll();
+
+            qApp->setStyleSheet(styles);
 
             // And save the new stylesheet path.
             if (currentStyle == QGC_MAINWINDOW_STYLE_LIGHT)
@@ -1094,14 +1111,18 @@ bool MainWindow::loadStyle(QGC_MAINWINDOW_STYLE style, QString cssFile)
             // And trigger any changes to other UI elements that are watching for
             // theme changes.
             emit styleChanged(style);
+        } else {
+            qDebug() << "Unable to load slave style sheet:" << cssFile;
+            success = false;
+            goto Error;
+        }
+    }
 
+Error:
     // Finally restore the cursor before returning.
     qApp->restoreOverrideCursor();
-        return true;
-    }
     
-    // Otherwise alert return a failure code.
-    return false;
+    return success;
 }
 
 /**