get_my/client/MainLayout.js

import { UserConfig } from "../imports/api/userConfig";

Template.MainLayout.onCreated(function() {
    this.subscribe("UserConfigPrefs");
});

Template.MainLayout.onRendered(function() {
    this.autorun(() => {
        let myId = Meteor.userId();
        const getConfig = async() => {
            let myprefs = await UserConfig.findOneAsync({ user: myId });
            try {
                if (!myprefs) {
                    // console.log("User Prefs appear undefined.");
                } else {
                    if (myprefs.darkMode == "light") {
                        // console.log("Found theme as light");
                        Session.set("myTheme", "light");
                        document.documentElement.setAttribute('theme', "light");
                    } else {
                        // console.log("Found theme as dark");
                        Session.set("myTheme", "dark");
                        document.documentElement.setAttribute('theme', "dark");
                    } 
                }
            } catch(error) {
                console.log("    ERROR getting user preferences: " + error);
            }
        }
        getConfig();
    });
});

Template.MainLayout.helpers({
    
});

Template.MainLayout.events({

});