chore: migrate deprecation code && code cleanup (#708)

Fixes all issues in `flutter analyze`.
<Reviewed>

Commits:
* chore: migrate deprecated text style

* chore: migrate `toggleableActiveColor` to individual theme

* chore: don't use 'BuildContext's across async gaps

lib/ui/widgets/shared/custom_sliver_app_bar.dart

@@ -33,7 +33,7 @@ class CustomSliverAppBar extends StatelessWidget {
           : IconButton(
               icon: Icon(
                 Icons.arrow_back,
-                color: Theme.of(context).textTheme.headline6!.color,
+                color: Theme.of(context).textTheme.titleLarge!.color,
               ),
               onPressed:
                   onBackButtonPressed ?? () => Navigator.of(context).pop(),