qt: Make UnitDisplayStatusBarControl aware of runtime palette change

This change fixes the GUI when changing appearance on macOS.

src/qt/bitcoingui.cpp

@@ -1450,9 +1450,10 @@ bool BitcoinGUI::isPrivacyModeActivated() const
     return m_mask_values_action->isChecked();
 }
 
-UnitDisplayStatusBarControl::UnitDisplayStatusBarControl(const PlatformStyle *platformStyle) :
-    optionsModel(nullptr),
-    menu(nullptr)
+UnitDisplayStatusBarControl::UnitDisplayStatusBarControl(const PlatformStyle *platformStyle)
+    : optionsModel(nullptr),
+      menu(nullptr),
+      m_platform_style{platformStyle}
 {
     createContextMenu();
     setToolTip(tr("Unit to show amounts in. Click to select another unit."));
@@ -1465,7 +1466,7 @@ UnitDisplayStatusBarControl::UnitDisplayStatusBarControl(const PlatformStyle *pl
     }
     setMinimumSize(max_width, 0);
     setAlignment(Qt::AlignRight | Qt::AlignVCenter);
-    setStyleSheet(QString("QLabel { color : %1 }").arg(platformStyle->SingleColor().name()));
+    setStyleSheet(QString("QLabel { color : %1 }").arg(m_platform_style->SingleColor().name()));
 }
 
 /** So that it responds to button clicks */
@@ -1474,6 +1475,18 @@ void UnitDisplayStatusBarControl::mousePressEvent(QMouseEvent *event)
     onDisplayUnitsClicked(event->pos());
 }
 
+void UnitDisplayStatusBarControl::changeEvent(QEvent* e)
+{
+#ifdef Q_OS_MACOS
+    if (e->type() == QEvent::PaletteChange) {
+        QString style = QString("QLabel { color : %1 }").arg(m_platform_style->SingleColor().name());
+        if (style != styleSheet()) {
+            setStyleSheet(style);
+        }
+    }
+#endif
+}
+
 /** Creates context menu, its actions, and wires up all the relevant signals for mouse events. */
 void UnitDisplayStatusBarControl::createContextMenu()
 {

src/qt/bitcoingui.h

@@ -334,10 +334,12 @@ public:
 protected:
     /** So that it responds to left-button clicks */
     void mousePressEvent(QMouseEvent *event) override;
+    void changeEvent(QEvent* e) override;
 
 private:
     OptionsModel *optionsModel;
     QMenu* menu;
+    const PlatformStyle* m_platform_style;
 
     /** Shows context menu with Display Unit options by the mouse coordinates */
     void onDisplayUnitsClicked(const QPoint& point);