add diagonal stripe pattern to klausur and aufgabe class in c course

- update klausur class with 135deg diagonal stripes (pink #fce4ec)
- add aufgabe class with solid background
- apply to all termin slides (1-3)

courses/223015c/slides/2025-12-20-termin-1-geschichte-grundlagen-html.md

@@ -49,11 +49,23 @@ a {
   color: var(--color-highlight);
 }
 section.klausur {
-  background: #fce4ec !important;
+  background: repeating-linear-gradient(
+    135deg,
+    #fce4ec,
+    #fce4ec 20px,
+    #fff 20px,
+    #fff 40px
+  ) !important;
 }
 section.klausur footer {
   display: none;
 }
+section.aufgabe {
+  background: #fce4ec !important;
+}
+section.aufgabe footer {
+  display: none;
+}
 </style>
 
 <!-- _class: invert -->

courses/223015c/slides/2026-01-10-termin-2-netzwerke-protokolle-css.md

@@ -49,11 +49,23 @@ a {
   color: var(--color-highlight);
 }
 section.klausur {
-  background: #fce4ec !important;
+  background: repeating-linear-gradient(
+    135deg,
+    #fce4ec,
+    #fce4ec 20px,
+    #fff 20px,
+    #fff 40px
+  ) !important;
 }
 section.klausur footer {
   display: none;
 }
+section.aufgabe {
+  background: #fce4ec !important;
+}
+section.aufgabe footer {
+  display: none;
+}
 </style>
 
 <!-- _class: invert -->

courses/223015c/slides/2026-01-24-termin-3-interaktivitaet-javascript.md

@@ -48,6 +48,24 @@ code {
 a {
   color: var(--color-highlight);
 }
+section.klausur {
+  background: repeating-linear-gradient(
+    135deg,
+    #fce4ec,
+    #fce4ec 20px,
+    #fff 20px,
+    #fff 40px
+  ) !important;
+}
+section.klausur footer {
+  display: none;
+}
+section.aufgabe {
+  background: #fce4ec !important;
+}
+section.aufgabe footer {
+  display: none;
+}
 </style>
 
 <!-- _class: invert -->