Frontend.Maui: Color.Default equivalent for Maui

Save previous assigned color and use instead of Color.Default.
This is needed because maui doesn't have Color.Default [1]. And
this commit is workaround for that.

[1] dotnet/Microsoft.Maui.Graphics#57

src/GWallet.Frontend.XF/SendPage.xaml.fs

@@ -72,7 +72,13 @@ type SendPage(account: IAccount, receivePage: Page, newReceivePageFunc: unit->Pa
     let transactionLayout = mainLayout.FindByName<StackLayout> "transactionLayout"
     let transactionLabel = mainLayout.FindByName<Label> "transactionLabel"
     let transactionEntry = mainLayout.FindByName<Entry> "transactionEntry"
+#if !XAMARIN
+    let defaultTransactionEntryTextColor = transactionEntry.TextColor
+#endif
     let amountToSendEntry = mainLayout.FindByName<Entry> "amountToSend"
+#if !XAMARIN
+    let defaultAmountToSendTextColor = amountToSendEntry.TextColor
+#endif
     let destinationAddressEntry = mainLayout.FindByName<Entry> "destinationAddressEntry"
     let allBalanceButton = mainLayout.FindByName<Button> "allBalance"
     let passwordEntry = mainLayout.FindByName<Entry> "passwordEntry"
@@ -458,6 +464,8 @@ type SendPage(account: IAccount, receivePage: Page, newReceivePageFunc: unit->Pa
                     MainThread.BeginInvokeOnMainThread(fun _ ->
 #if XAMARIN
                         transactionEntry.TextColor <- Color.Default
+#else
+                        transactionEntry.TextColor <- defaultTransactionEntryTextColor
 #endif
                         destinationAddressEntry.Text <- unsignedTransaction.Proposal.DestinationAddress
                         amountToSendEntry.Text <- unsignedTransaction.Proposal.Amount.ValueToSend.ToString()
@@ -485,6 +493,8 @@ type SendPage(account: IAccount, receivePage: Page, newReceivePageFunc: unit->Pa
                     MainThread.BeginInvokeOnMainThread(fun _ ->
 #if XAMARIN
                         transactionEntry.TextColor <- Color.Default
+#else
+                        transactionEntry.TextColor <- defaultTransactionEntryTextColor
 #endif
                         sendOrSignButton.IsEnabled <- true
                     )
@@ -539,6 +549,10 @@ type SendPage(account: IAccount, receivePage: Page, newReceivePageFunc: unit->Pa
                         MainThread.BeginInvokeOnMainThread(fun _ ->
                             amountToSend.TextColor <- Color.Default
                         )
+#else
+                        MainThread.BeginInvokeOnMainThread(fun _ ->
+                            amountToSend.TextColor <- defaultAmountToSendTextColor
+                        )
 #endif
                         match usdRate with
                         | NotFresh NotAvailable ->