docs/source/_templates/custom-scripts.html

<script>
    // Dark mode and read mode detection

    // Dark mode
    var isDarkMode = localStorage.getItem("dark-mode");
    var dmToggleBtn = document.getElementById("dark-mode-toggle");
    if (isDarkMode === "true") {
       document.documentElement.classList.add("dark-mode");
       dmToggleBtn.classList.add("active");
    }

    // Read mode
    var isReadMode = localStorage.getItem("read-mode");
    var rmToggleBtn = document.getElementById("read-mode-toggle");

    if (isReadMode === "true") {
        document.documentElement.classList.add("read-mode");
        rmToggleBtn.classList.add("active");
    }
 </script>