Constant THEME_PRELOAD_SCRIPT 

const THEME_PRELOAD_SCRIPT: &str = r#"
(function() {
    var theme = 'system';
    try {
        var saved = localStorage.getItem('yggdrasil-theme');
        if (saved === 'dark' || saved === 'light') theme = saved;
    } catch (e) {}
    document.documentElement.setAttribute('data-theme-mode', theme);
    document.documentElement.classList.toggle('dark',
        theme === 'dark' || (theme === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches));
})();
"#;