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/223015c/slides/assets/klausur-pattern.svg

@@ -1,9 +1,4 @@
 <svg xmlns="http://www.w3.org/2000/svg" width="80" height="80">
-  <defs>
-    <pattern id="diagonalStripes" patternUnits="userSpaceOnUse" width="80" height="80" patternTransform="rotate(135)">
-      <rect width="40" height="80" fill="#fce4ec"/>
-      <rect x="40" width="40" height="80" fill="#ffffff"/>
-    </pattern>
-  </defs>
-  <rect width="100%" height="100%" fill="url(#diagonalStripes)"/>
+  <rect width="80" height="80" fill="#fce4ec"/>
+  <path d="M-20,20 l40,-40 M0,80 l80,-80 M60,100 l40,-40" stroke="#ffffff" stroke-width="40"/>
 </svg>