diff --git a/templates/base/head.tmpl b/templates/base/head.tmpl
index 50253038ed..70a88ff755 100644
--- a/templates/base/head.tmpl
+++ b/templates/base/head.tmpl
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html lang="{{.locale.Lang}}" class="theme-{{.SignedUser.Theme}}">
+<html lang="{{.locale.Lang}}" class="theme-{{if .SignedUser.Theme}}{{.SignedUser.Theme}}{{else}}auto{{end}}">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">