Create Javascript localStorage for website background change

Hello and big thanks for a all S.O community

I come today because i try to create local storage with Javascript for my users can change background and page settings on my website and find it back when page reload.
I tried many attempt but i’m so beginner for do make it work

    var m = new s.a.Store({
    modules: {
        programs: p,
        preferences: {
            state: {
                background: "images/backgrounds/background2.jpg",
                preferredColor: "#1e90ff",
                isWiFiConnected: !1
            },
            getters: {
                background: function(t) {
                    return t.background
                },
                preferredColor: function(t) {
                    return t.preferredColor
                },
                isWiFiConnected: function(t) {
                    return t.isWiFiConnected
                }
            },
            actions: {},
            mutations: {
                changePreferredColor: function(t, e) {
                    t.preferredColor = e
                },
                changeBackground: function(t, e) {
                    t.background = e
                },
                toggleWiFiConnection: function(t) {
                    t.isWiFiConnected = !t.isWiFiConnected
                }
            }
        },
        filesystem: d
    },
    strict: !1
});
i.a.prototype.$t = function(t) {
    return t
}, i.a.config.devtools = !0, new i.a({
    el: "#app",
    store: m,
    components: {
        App: a.a
    }
})
},
function(t, e, n) {
    "use strict";
    (function(e, n) {
            var r = Object.freeze({});

            function i(t) {
                return null == t
            }

            function o(t) {
                return null != t
            }

            function a(t) {
                return !0 === t
            }

            function s(t) {
                return "string" == typeof t || "number" == typeof t || "symbol" == typeof t || "boolean" == typeof t
            }

            function u(t) {
                return null !== t && "object" == typeof t
            }

 there is the code i tried to add just before without sucess (blank page) :

        var m = { ...localStorage };
    localStorage.getItem("background");
    localStorage.setItem("background"); 

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Thanks for your support

Best regards