Set standard base Qt style on Windows and macOS

This sets the "Fusion" Qt theme as the application's base style for
Windows and macOS to normalize some minor appearance issues between
different platforms. Fusion is the default style used for Linux for
desktops without a custom window manager theme.

From what I read, the Windows theme still has some high-DPI issues,
and the Fusion styles support high-DPI displays very well.

Author: cyrossignol

src/qt/bitcoin.cpp

@@ -241,7 +241,7 @@ int main(int argc, char *argv[])
 
     // Generate high-dpi pixmaps
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
-#if QT_VERSION >= 0x050600 && !defined(WIN32)
+#if QT_VERSION >= 0x050600
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 #endif
 
@@ -264,6 +264,20 @@ int main(int argc, char *argv[])
     else
         app.setApplicationName("Gridcoin-Qt");
 
+#if defined(Q_OS_WIN) || defined(Q_OS_MAC)
+    // Apply Qt's built-in "Fusion" theme as the application's base styles to
+    // normalize layout discrepancies between platforms and fix some high-DPI
+    // scaling issues on Windows. Gridcoin uses highly-customized stylesheets
+    // which obscure most of the platform's styles anyway. That said, respect
+    // the presence of Qt's "-style" option to bypass this if necessary. Skip
+    // the override on Linux for now so that a user's window manager Qt theme
+    // comes through for widgets without an explicit application style.
+    //
+    if (!IsArgSet("-style")) {
+        app.setStyle("Fusion");
+    }
+#endif
+
     // Install global event filter that makes sure that long tooltips can be word-wrapped
     app.installEventFilter(new GUIUtil::ToolTipToRichTextFilter(TOOLTIP_WRAP_THRESHOLD, &app));