qt: use system accent color with Qt >= 6.6 in `SystemPaletteThemeProvider`

Request review @chub.

modules/gui/qt/style/systempalettethemeprovider.cpp

@@ -135,9 +135,13 @@ static int updatePalette(vlc_qt_theme_provider_t* obj)
     auto sys = static_cast<SystemePaletteObserver*>(obj->p_sys);
 
     QPalette& palette = sys->m_palette;
-
+#if QT_VERSION >= QT_VERSION_CHECK(6, 6, 0)
+    QColor accent = palette.color(QPalette::Normal, QPalette::Accent);
+    QColor accentPressed = accent.lighter(150);
+#else
     QColor accent = QColor( sys->m_isDark ? "#FF8800" : "#FF610A" );
     QColor accentPressed = QColor( sys->m_isDark ? "#e67a30" : "#e65609" );
+#endif
     QColor textOnAccent = Qt::white;
     QColor visualFocus =  sys->m_isDark ? Qt::white : Qt::black;
     QColor shadow = palette.color(QPalette::Normal, QPalette::Shadow);