fix klausur pdf backgrounds with _backgroundColor directive

- remove !important from klausur css gradient (allows directive override)
- add _backgroundColor directive to all klausur slides for pdf
- web: shows css gradient stripes
- pdf: shows solid color background

courses/223015b/slides/2026-01-23-termin-3-speichermedien-schnittstellen.md

@@ -55,13 +55,7 @@ section.klausur {
     #e3f2fd 40px,
     #fff 40px,
     #fff 80px
-  ) !important;
-}
-@media print {
-  section.klausur {
-    background: url('./assets/klausur-pattern.svg') !important;
-    background-size: cover !important;
-  }
+  );
 }
 section.aufgabe {
   background: #e3f2fd !important;