Change theme and store it in local storage

I have two themes "dark" and "light", and I want the theme to change when clicking the checkbox.

This is how I changed the theme:

document.documentElement.setAttribute('data-theme', 'dark');

And now, this is working. But I want this change to be saved in local storage, thus even after reloading the page, the theme to stay as it is without changing.

here is my code:

checkBox.addEventListener('change', function () {
    if(this.checked) {

        document.documentElement.setAttribute('data-theme', 'dark');
        localStorage.setItem( 'data-theme', 'dark');   
    }
    else {
        document.documentElement.setAttribute('data-theme', 'light');
        localStorage.setItem('data-theme', 'light');
    }
})

Did I made a mistake or is there something I don't understand?