libreevent/src/server/ui/home/helper.js

var theme = localStorage.getItem( 'theme' ) ?? '';
if ( window.matchMedia( '(prefers-color-scheme: dark)' ).matches || theme === '☼' ) {
    document.documentElement.classList.add( 'dark' );
    document.getElementById( 'themeSelector' ).innerHTML = '☼';
    theme = '☼';
} else {
    document.documentElement.classList.add( 'light' );
    document.getElementById( 'themeSelector' ).innerHTML = '☽';
    theme = '☽';
}

function changeTheme () {
    if ( theme === '☼' ) {
        document.documentElement.classList.remove( 'dark' );
        document.documentElement.classList.add( 'light' );
        localStorage.setItem( 'theme', '☽' );
        document.getElementById( 'themeSelector' ).innerHTML = '☽';
        theme = '☽';
    } else if ( theme === '☽' ) {
        document.documentElement.classList.remove( 'light' );
        document.documentElement.classList.add( 'dark' );
        localStorage.setItem( 'theme', '☼' );
        document.getElementById( 'themeSelector' ).innerHTML = '☼';
        theme = '☼';
    }
}