Fix issues with white theme & Forez join group screen

lib/ui/widgets/group_membership_widget.dart

@@ -140,6 +140,7 @@ class _GroupMembershipWidgetState extends SafeState<GroupMembershipWidget> {
     return RichText(
       text: TextSpan(
           text: tr("Request membership"),
+          style: blackForWhiteTheme(),
           recognizer: TapGestureRecognizer()
             ..onTap = () => _requestMembership()),
     );