To reproduce:

Set your theme to Windows8:

ThemeResolutionService.ApplicationThemeName = new Windows8Theme().ThemeName;

Add a RadTextBox and RadMaskedEditBox:

RadTextBox textBox = new RadTextBox
{
    Parent = this,
    Dock = DockStyle.Top,
    Text = "TextBox"
};

RadMaskedEditBox maskedEditBox = new RadMaskedEditBox
{
    Parent = this,
    Dock = DockStyle.Top,
    Text = "MaskedEditBox"
};

RadDateTimePicker dateTimePicker = new RadDateTimePicker
{
    Parent = this,
    Dock = DockStyle.Top,
    Text = "DateTimePicker"
};


You can also set their Enabled property to false prior running the form. Run the form and you will see that their background is black.

Workaround:

For the RadTextBox, set the BackColor of the RootElement to white:

textBox.RootElement.BackColor = Color.White;

For the RadMaskedEditBox, set the BackColor of the TextBoxItem to white:

maskedEditBox.MaskedEditBoxElement.TextBoxItem.BackColor = Color.White;

For the RadDateTimePicker, set the BackColor of the TextBoxItem to white as follows:

dateTimePicker.DateTimePickerElement.TextBoxElement.TextBoxItem.BackColor = Color.White;