I would like to allow visitors who don't have dark mode enabled on their device or browser a one-click option to enable it.

How do I fetch current cookie, change it, and then reset it before early in the loading stage of the site?

const currentScheme = () => {
    var colorScheme = "";           
    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {               
        colorScheme = "dark";
    }
    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => {
        colorScheme = e.matches ? "dark" : "light";
        if(colorScheme !== "dark") {                
            if(document.body.classList.contains("global-hash-dark-version")) {
                document.body.classList.remove("global-hash-dark-version");
            }
        } else {
            if(!document.body.classList.contains("global-hash-dark-version")) {
                document.body.classList.add("global-hash-dark-version");
            }
        }
    })
    return colorScheme;
}

function getCookie(name) {
        var nameEQ = name + "=";
        var ca = document.cookie.split(';');
        for(var i=0;i < ca.length;i++) {
                var c = ca[i];
                while (c.charAt(0)==' ') c = c.substring(1,c.length);
                if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
        }
        return null;
}
let cookieValue = getCookie("choice_settings");
if(cookieValue) {
    let currentSettings = JSON.parse(cookieValue);
    if (currentSettings.colorScheme == "dark") {
            document.body.classList.add("global-hash-dark-version");
            console.log("dark")
    }                               
} else {
    const temp_scheme = currentScheme();
    if (temp_scheme == "dark") {    
        currentSettings = {
            colorScheme: "dark", 
            ...currentSettings
        }

//  This is where I am stuck. 
//  How do I fetch, update, and then reset the cookie with settings?
//  Other settings will be things like privacy consent and dismissed
//  notifications
//  
//  

}