[For 1.8] day and night mode cookies

[Anka06]

global.css 

.switcher {
  height: auto;
  color:red
}

.switcher a {
  font-size: 9px;
  font-weight: 600;
  letter-spacing: 1px;
  font-style: normal;
  line-height: 1;
  padding: 5px 10px;
  color: red;
  border-radius: 2px
  cursor: pointer;
}

.switcher a:hover {
  color: green;
}

.pageskin .pageskin-remove,
.switcher a {
  text-transform: uppercase;
  cursor: pointer;

}

.switcher-mode-on {
   cursor: pointer;
   color:blue;
}
	
.switcher-mode-off {
  cursor: pointer;
  color:red;
}

.ff-hide,
.hide {
  display: none !important;
}

Header find

<a href="{$mybb->settings['bburl']}/misc.php?action=help" class="help">{$lang->toplinks_help}</a></li>

add below

<a class="switcher-mode-on hide"><i class="fas fa-moon"title="Open Game Mode"></i></a> <a class="switcher-mode-off hide"><i class="far fa-moon"title="Close Game Mode"></i></a>

headerinclude

<script type="text/javascript" src="{$mybb->asset_url}/images/moonmode.js"></script>

{$stylesheets}

<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.7.1/css/all.css"/>

style-dark.css edit

Upload to ftp images folder

[Crazycat]

js and css files in images/ ?
Why don't you use the correct directories ?

[Dr_The_One]

Please edit your first post.
Where and what to add in headerinclude.
Thank you .

[8guawong]

this work for all theme?

[Devilshakerz]

A similar plugin that additionally modifies stylesheets and editortheme server-side to avoid the flash of unstyled content: DVZ Theme Options (documentation: https://github.com/dvz/mybb-dvzThemeOptions)