[Web] dark mode logo support

Signed-off-by: Kristian Feldsam <feldsam@gmail.com>

data/web/inc/header.inc.php

@@ -40,6 +40,7 @@ $globalVariables = [
   'ui_texts' => $UI_TEXTS,
   'css_path' => '/cache/'.basename($CSSPath),
   'logo' => customize('get', 'main_logo'),
+  'logo_dark' => customize('get', 'main_logo_dark'),
   'available_languages' => $AVAILABLE_LANGUAGES,
   'lang' => $lang,
   'skip_sogo' => (getenv('SKIP_SOGO') == 'y'),