<header class="header-contents">

    </header>
    <main class="main-contents">
        <label>
            <input id="theme" type="checkbox">
            <span>ダークテーマ</span>
        </label>
    </main>
    <footer class="footer-contents">

    </footer>
:root {
    --main-color: #5555ff;
    --base-color: #eee;
    --text-color: #333;
    --contents-width:500px;
}

:root.dark {
    --main-color: #333;
    --base-color: #000;
    --text-color: #fff;
}

* {
    box-sizing: border-box;
}

body {
    background-color: var(--base-color);
    color: var(--text-color);
}

.header-contents {
    max-width: calc(var(--contents-width));
    background-color: var(--main-color);
    width: 100%;
    height: 50px;
    margin: auto;
}

.main-contents {
    max-width: var(--contents-width);
    padding: 30px;
    width: 100vw;
    height: 100px;
    margin: auto;
}

.footer-contents {
    max-width: calc(var(--contents-width));
    background-color: var(--main-color);
    width: 100%;
    height: 100px;
    margin: auto;
}
const inpTheme = document.getElementById("theme");
const html = document.documentElement;
inpTheme.addEventListener("change", ()=>{
    if(!inpTheme.checked) {
        html.classList.remove("dark");
    }
    else {
        html.classList.add("dark");
    }
});

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External JavaScript

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