add diagonal stripe pattern to klausur and aufgabe class across b course

- update klausur class with 135deg diagonal stripes
- add aufgabe class with solid background
- apply to all termin slides (0-5)

courses/223015b/slides/2025-12-19-termin-0-intro.md

@@ -49,11 +49,23 @@ a {
   color: var(--color-highlight);
 }
 section.klausur {
-  background: #e3f2fd !important;
+  background: repeating-linear-gradient(
+    135deg,
+    #e3f2fd,
+    #e3f2fd 20px,
+    #fff 20px,
+    #fff 40px
+  ) !important;
 }
 section.klausur footer {
   display: none;
 }
+section.aufgabe {
+  background: #e3f2fd !important;
+}
+section.aufgabe footer {
+  display: none;
+}
 </style>
 
 <!-- _class: invert -->