How can I securely store and access user preferences in the browser using localStorage or sessionStorage?

<!-- BEGIN COPY / PASTE -->
        // Function to save user preferences
        function savePreferences(key, value) {
            const jsonData = JSON.stringify(value);
            localStorage.setItem(key, jsonData);
        }

        // Function to load user preferences
        function loadPreferences(key) {
            const jsonData = localStorage.getItem(key);
            return jsonData ? JSON.parse(jsonData) : null;
        }

        // Example usage
        const userPreferences = {
            theme: 'dark',
            fontSize: '16px'
        };

        // Save preferences
        savePreferences('userPreferences', userPreferences);

        // Load preferences
        const loadedPreferences = loadPreferences('userPreferences');
        console.log(loadedPreferences);
        <!-- END COPY / PASTE -->