Fix TextBoxBase related BackColor issues (#13820)

related to  #13813


## Proposed changes

- add a condition whether there is a specified color or not


## fixed Controls
TextBox, DomainUpDown, NumericUpDown, RichTextBox, MaskedTextBox 

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## Customer Impact

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## Regression? 

- Yes / No

## Risk

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## Screenshots <!-- Remove this section if PR does not change UI -->

### Before

<img width="516" height="549" alt="image"
src="https://github.com/user-attachments/assets/4e48b944-20bd-4ddf-86e7-c41ba3256cd5"
/>

### After

<img width="568" height="553" alt="image"
src="https://github.com/user-attachments/assets/228983f4-92da-4f81-97a2-455411b038e2"
/>


## Test methodology <!-- How did you ensure quality? -->

- manual

Author: KlausLoeffelmann

src/System.Windows.Forms/System/Windows/Forms/Controls/TextBox/TextBoxBase.cs

@@ -949,7 +949,8 @@ private void EnsureReadonlyBackgroundColor(bool value)
     {
         // If we have no specifically defined back color, we set the back color in case we're in dark mode.
         if (Application.IsDarkModeEnabled
-            && DarkModeRequestState is true)
+            && DarkModeRequestState is true
+            && !ShouldSerializeBackColor())
         {
             base.BackColor = value ? SystemColors.ControlLight : SystemColors.Window;
             Invalidate();