Make theme key accessible via CSS

internal/glance/templates/document.html

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en" id="top" data-scheme="{{ if .Request.Theme.Light }}light{{ else }}dark{{ end }}">
+<html lang="en" id="top" data-theme="{{ .Request.Theme.Key }}" data-scheme="{{ if .Request.Theme.Light }}light{{ else }}dark{{ end }}">
 <head>
     {{ block "document-head-before" . }}{{ end }}
     <script>