Fix issues with white theme & Forez join group screen

lib/ui/screens/group_access_denied_screen.dart

@@ -68,7 +68,10 @@ class _GroupAccessDeniedScreenState extends SafeState<GroupAccessDeniedScreen> {
               style: TextStyle(fontSize: 20),
             ),
             Spacer(),
-            Text(tr("A registration is required to access this group page.")),
+            Text(
+              tr("A registration is required to access this group page."),
+              textAlign: TextAlign.center,
+            ),
             Spacer(),
             GroupMembershipWidget(
               group: _group,

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()),
     );