fix klausur slide pdf rendering with @media print fallback

courses/223015b/slides/02-bild-audio-video.md

@@ -57,6 +57,11 @@ section.klausur {
     #fff 80px
   ) !important;
 }
+@media print {
+  section.klausur {
+    background: #e3f2fd !important;
+  }
+}
 </style>
 
 <!-- _class: invert -->