Update header.ut

luci-theme-glass/ucode/template/themes/glass/header.ut

@@ -63,7 +63,7 @@
   }
 -%}
 <!DOCTYPE html>
-<html lang="{{ dispatcher.lang }}">
+<html lang="{{ dispatcher.lang }}"{% if (glass_mode == 'dark'): %} data-darkmode="true"{% elif (glass_mode == 'light'): %} data-darkmode="false"{% endif %}>
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no">