Fix dark theme dialog colors. (#569)

By using `ColorScheme.fromSwatch().copyWith(...)` the `ColorScheme` was
tailored to light-mode which caused some dialogs to be displayed in the
wrong colors when using dark mode.
Specifying the `Brightness` inside `ColorScheme.fromSeed` fixed this
issue.

|  Before  | After  |
|---|---|
|
![grafik](https://user-images.githubusercontent.com/29028262/230205421-2ef13e8c-a44e-4b19-88f7-5ecaeb40198b.png)
|
![grafik](https://user-images.githubusercontent.com/29028262/230205211-dc9b81a2-1fe2-4d0f-840f-0f77cb715be7.png)
|
|
![grafik](https://user-images.githubusercontent.com/29028262/230205664-9fd60636-9b37-4f1a-84a0-1192c6866da5.png)
|
![grafik](https://user-images.githubusercontent.com/29028262/230205806-05d587ad-e4f9-49b3-9c9b-3734ec487d26.png)
|
|![grafik](https://user-images.githubusercontent.com/29028262/230208871-f0afa130-509e-471c-ac0e-923f92a108fc.png)
|
![grafik](https://user-images.githubusercontent.com/29028262/230209102-81cab437-db16-4ac0-909e-73f87d2892e1.png)
|

|![grafik](https://user-images.githubusercontent.com/29028262/230209346-c150f119-25d3-41ae-a3cd-7471ea762982.png)
|
![grafik](https://user-images.githubusercontent.com/29028262/230208730-adfa9caa-d812-4c3d-9b0e-a165660d981d.png)

Fixes #167 
Fixes #168
Fixes #169
Fixes #384 
Fixes #496

app/lib/widgets/apple_sign_in_button.dart

@@ -72,6 +72,7 @@ class SignInWithAppleButton extends StatelessWidget {
                   child: Icon(
                     FontAwesomeIcons.apple,
                     size: 26,
+                    color: Colors.white,
                   ),
                 ),
                 Padding(

lib/sharezone_widgets/lib/src/theme/brightness/dark_theme.dart

@@ -42,14 +42,15 @@ final darkTheme = ThemeData(
       .copyWith(headlineMedium: const TextStyle(color: Colors.white)),
   pageTransitionsTheme: _pageTransitionsTheme,
   snackBarTheme: _snackBarTheme.copyWith(
-    contentTextStyle: const TextStyle(
-      color: Colors.white,
-    ),
+    contentTextStyle: const TextStyle(color: Colors.white),
+    backgroundColor: Colors.black,
   ),
   bottomSheetTheme: _bottomSheetTheme,
   dialogTheme: _dialogTheme,
-  colorScheme: ColorScheme.fromSwatch()
-      .copyWith(secondary: _accentColor, brightness: Brightness.dark),
+  colorScheme: ColorScheme.fromSeed(
+    seedColor: primaryColor,
+    brightness: Brightness.dark,
+  ),
   tabBarTheme: const TabBarTheme(labelColor: Colors.white),
   checkboxTheme: CheckboxThemeData(
     fillColor: