Это ответ.
<Style TargetType="{x:Type y:XamTextEditor}">
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding Path=ValueConstraint.Nullable, RelativeSource={RelativeSource Self}}" Value="false" />
<Condition Binding="{Binding Path=IsReadOnly, RelativeSource={RelativeSource Self}}" Value="false" />
<Condition Binding="{Binding Path=IsEnabled, RelativeSource={RelativeSource Self}}" Value="true" />
<Condition Binding="{Binding Path=Visibility, RelativeSource={RelativeSource Self}}" Value="Visible" />
</MultiDataTrigger.Conditions>
<Setter Property="Background" Value="red"></Setter>
</MultiDataTrigger>
</Style.Triggers>
</Style>