Fix using wrong icon color for 'Benutzerdefiniert' button in dark mode (

#1262)

| Before | After |
|--------|--------|
| <img width="1086" alt="image"
src="https://github.com/SharezoneApp/sharezone-app/assets/24459435/7f46c618-b18b-4088-9ff0-7678ce609d1d">
| <img width="1086" alt="Screenshot 2024-01-18 at 16 38 20"
src="https://github.com/SharezoneApp/sharezone-app/assets/24459435/298b2118-65b4-4c1a-ba2e-2bb88fae59f2">
|

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Style**
- Updated the visual appearance of the due date chips to match the
theme's icon color.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/lib/homework/homework_dialog/homework_dialog.dart

@@ -665,7 +665,10 @@ class _DueDateChipsState extends State<_DueDateChips> {
                         ),
                       ),
                     InputChip(
-                      avatar: const Icon(Icons.edit),
+                      avatar: Icon(
+                        Icons.edit,
+                        color: Theme.of(context).iconTheme.color,
+                      ),
                       label: const Text('Benutzerdefiniert'),
                       onPressed: lessonChipsSelectable
                           ? () async {