010101 You're right, although the custom header does satisfy most needs unless it's specific - like changing the color of the Chrome header on Android and Safari - if you attempt to do this in the "conventional way" in the GUI, it's added too late in the header and overwritten. To work around this, you'd need to configure extend.php in your flarum root such as the below
use Flarum\Extend;
use Flarum\Frontend\Document;
return [
(new Extend\Frontend('forum'))
->content(function (Document $document) {
$document->meta['theme-color'] = '#12171a';
})
];