qt: Make PlatformStyle aware of runtime palette change

This change is a prerequisite to support changeable appearance on macOS.

src/qt/platformstyle.cpp

@@ -71,25 +71,28 @@ PlatformStyle::PlatformStyle(const QString &_name, bool _imagesOnButtons, bool _
     name(_name),
     imagesOnButtons(_imagesOnButtons),
     colorizeIcons(_colorizeIcons),
-    useExtraSpacing(_useExtraSpacing),
-    singleColor(0,0,0),
-    textColor(0,0,0)
+    useExtraSpacing(_useExtraSpacing)
+{
+}
+
+QColor PlatformStyle::TextColor() const
+{
+    return QApplication::palette().color(QPalette::WindowText);
+}
+
+QColor PlatformStyle::SingleColor() const
 {
-    // Determine icon highlighting color
     if (colorizeIcons) {
         const QColor colorHighlightBg(QApplication::palette().color(QPalette::Highlight));
         const QColor colorHighlightFg(QApplication::palette().color(QPalette::HighlightedText));
         const QColor colorText(QApplication::palette().color(QPalette::WindowText));
         const int colorTextLightness = colorText.lightness();
-        QColor colorbase;
-        if (abs(colorHighlightBg.lightness() - colorTextLightness) < abs(colorHighlightFg.lightness() - colorTextLightness))
-            colorbase = colorHighlightBg;
-        else
-            colorbase = colorHighlightFg;
-        singleColor = colorbase;
+        if (abs(colorHighlightBg.lightness() - colorTextLightness) < abs(colorHighlightFg.lightness() - colorTextLightness)) {
+            return colorHighlightBg;
+        }
+        return colorHighlightFg;
     }
-    // Determine text color
-    textColor = QColor(QApplication::palette().color(QPalette::WindowText));
+    return {0, 0, 0};
 }
 
 QImage PlatformStyle::SingleColorImage(const QString& filename) const

src/qt/platformstyle.h

@@ -21,8 +21,8 @@ public:
     bool getImagesOnButtons() const { return imagesOnButtons; }
     bool getUseExtraSpacing() const { return useExtraSpacing; }
 
-    QColor TextColor() const { return textColor; }
-    QColor SingleColor() const { return singleColor; }
+    QColor TextColor() const;
+    QColor SingleColor() const;
 
     /** Colorize an image (given filename) with the icon color */
     QImage SingleColorImage(const QString& filename) const;
@@ -43,9 +43,6 @@ private:
     bool imagesOnButtons;
     bool colorizeIcons;
     bool useExtraSpacing;
-    QColor singleColor;
-    QColor textColor;
-    /* ... more to come later */
 };
 
 #endif // BITCOIN_QT_PLATFORMSTYLE_H