[v3 mac] Add IsDarkMode to application and JS runtime. Add Common.ThemeChanged event

v3/pkg/application/messageprocessor_system.go

@@ -0,0 +1,18 @@
+package application
+
+import (
+	"net/http"
+)
+
+func (m *MessageProcessor) processSystemMethod(method string, rw http.ResponseWriter, r *http.Request, window *WebviewWindow, params QueryParams) {
+
+	switch method {
+	case "IsDarkMode":
+		m.json(rw, globalApplication.IsDarkMode())
+	default:
+		m.httpError(rw, "Unknown system method: %s", method)
+	}
+
+	m.Info("Runtime:", "method", "System."+method)
+
+}