Hi everybody, 
On this tutorial I want to show you, how to create a Night Mode on you're forum, to do this we need use a HTML widget in the Brand Header (global/brand-header).
Step 1
Create a button as in the gif 
<nav>
<div class="theme-switch-wrapper d-flex align-self-center">
<label class="theme-switch" for="checkbox">
<input type="checkbox" id="checkbox" />
<div class="slider border rounded-pill">
<div class="moon-sun p-1">
<i class="fa-solid fa-moon text-white float-start"></i>
<i class="fa-solid fa-sun text-warning float-end"></i>
</div></div>
</label>
</div>
</nav>
Step 2
We create a CSS style for the night mode 
<style>
/* css for night mode */
[data-theme="dark"] {
--font-color: #e1e1ff;
--bs-body-bg: #202124;
--bs-body-color: #f1f3f4;
--bs-body-color-rgb: 255,255,255;
--bs-nav-link-color: #fff;
--bs-border-color: #35363a;
--bs-body-bg-rgb: 32,33,36;
--bs-light-rgb: 32,33,36;
}
[data-theme="dark"] hr { border-top-color: #262729; }
[data-theme="dark"] .active .chat-room-btn { background-color: #262729; }
[data-theme="dark"] { .btn-outline { border-color: #b6b6b6; } }
[data-theme="dark"] { .btn-outline.active, .btn-outline:hover, .btn-link.active, .btn-link:hover { background-color: #35363a; } }
[data-theme="dark"] { .btn-ghost.active, .btn-ghost:hover, .btn-ghost-sm.active, .btn-ghost-sm:hover { background-color: #262729; } }
[data-theme="dark"] .skin-noskin .composer { background-color: #262729!important; }
[data-theme="dark"] .skin-noskin .bottombar-nav .dropdown-menu { background-color: #262729 !important; }
[data-theme="dark"] .bottombar-nav .nav-text { color: #ffffff; }
[data-theme="dark"] .composer .resizer { background: linear-gradient(transparent,#262729); }
[data-theme="dark"] .skin-noskin .bottombar-nav { background-color: #262729!important; }
[data-theme="dark"] .page-topic .pagination-block.ready { background-color: #202124 !important; border-color: #262729 !important; }
[data-theme="dark"] .form-control { background-color: #262729; border: 1px solid #35363a; color: #b6b6b6; }
[data-theme="dark"] .form-control:focus { color: white; background-color: #262729; border-color: #262729; box-shadow: inset 0px 0px 0px rgba(0,0,0,.075), 0 0 0 0.05rem rgb(27 115 249); }
[data-theme="dark"] .dropdown-item:hover { color: white; background-color: #262729; }
[data-theme="dark"] .tag-list .tag { background-color: #262729; color: white; }
[data-theme="dark"] .breadcrumb .breadcrumb-item span { color: #b6b6b6; }
[data-theme="dark"] .card { --bs-card-cap-bg: rgba(38,39,41); background-color: #262729; }
[data-theme="dark"] .skin-noskin nav.sidebar { background-color: #202124!important; }
[data-theme="dark"] .sidebar .nav-link.active { background-color: #35363a; }
[data-theme="dark"] { .sidebar .nav-link:focus, .nav-link:hover { background-color: #35363a !important; color: white; } }
[data-theme="dark"] .sticky-tools { background-color: #202124; }
[data-theme="dark"] .text-muted { color: #b6b6b6!important; }
[data-theme="dark"] nav-btn:hover { background-color: #35363a; }
[data-theme="dark"] .btn-ghost-sm:hover { background-color: #35363a; }
[data-theme="dark"] .btn:hover { background-color: #202124; border-color: #62656f; }
[data-theme="dark"] { .dropdown-menu, .dropdown-item { background-color: #202124; color: #f1f3f4; } }
[data-theme="dark"] .border-gray-300 { border-color: #35363a!important; }
[data-theme="dark"] ul.topics-list li.selected { background-color: #262729; }
[data-theme="dark"] .modal-content { background-color: #35363a; }
[data-theme="dark"] .account .avatar-wrapper { border: 4px solid #262729; }
@media (min-width: 576px) {
[data-theme="dark"] .topic .pagination-block .scroller-container { border-left: 2px solid #35363a; }
[data-theme="dark"] .page-topic .topic .posts.timeline>[component=post] { border-left: 2px solid #35363a; }
[data-theme="dark"] .page-topic .topic .posts.timeline>[component=post]:first-child:before { background-color: #35363a; }
[data-theme="dark"] .page-topic .topic .posts.timeline>[component=post]:last-child:after { background-color: #35363a; }
}
/* css for the switch a night/light */
.theme-switch {
display: inline-block;
height: 26px;
position: relative;
width: 55px;
}
.theme-switch input { display:none; }
.slider {
background-color: #ffffff;
bottom: 0;
cursor: pointer;
left: 0;
position: absolute;
right: 0;
top: 0;
transition: .4s;
}
.slider:before {
background-color: #1b73f9;
bottom: 3px;
content: "";
height: 18px;
width: 18px;
left: 4px;
position: absolute;
transition: .4s;
border-radius: 50px;
}
input:checked + .slider { background-color: #262729; }
input:checked + .slider:before { transform: translateX(28px); }
</style>
To change logo in dark mode add this code to css:
[data-theme="dark"] [component="brand/logo"] { content: url(/link/to/logo-for-dark-mode.png); }
This style was created for the Harmony theme without modifications, if you use custom style, you need change this css for you're personal theme style.
Step 3
Now create a JavaScript to switch a night/light mode and save choice in browser LocalStorage if user was refresh the page.
<script>
const toggleSwitch = document.querySelector('.theme-switch input[type="checkbox"]');
const currentTheme = localStorage.getItem('theme');
if (currentTheme) {
document.documentElement.setAttribute('data-theme', currentTheme);
if (currentTheme === 'dark') {
toggleSwitch.checked = true;
}
}
function switchTheme(e) {
if (e.target.checked) {
document.documentElement.setAttribute('data-theme', 'dark');
localStorage.setItem('theme', 'dark');
}
else { document.documentElement.setAttribute('data-theme', 'light');
localStorage.setItem('theme', 'light');
}
}
toggleSwitch.addEventListener('change', switchTheme, false);
</script>
That's all.
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