How can I set default value of property based on theme in MAUI

You can use AppThemeBinding and the provided Extensions for that.

First, you need to remove the binding to TextColor, because this will need to be set from the code-behind, and also give the Label a name using the x:Name attribute:

<ContentView xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="EpexMauiApp.Components.LabeledValue"
             x:Name="self">
        <Label x:Name="MyLabel" Text="Dummy text" FontAttributes="Bold"/>
</ContentView>

Then, in your code-behind, you need to set the AppThemeBinding programmatically in the constructor:

public LabeledValue()
{
    InitializeComponent();
    
    //set default values for AppThemeBinding
    MyLabel.SetAppThemeColor(Label.TextColorProperty, Colors.Blue, Colors.White);
}

Finally, you need to add a property changed handler that sets the TextColor when the user overrides the default setting:

public partial class LabeledValue : ContentView 
{
    public Color ValueColor
    {
        get { return (Color)GetValue(ValueColorProperty); }
        set { SetValue(ValueColorProperty, value); }
    }

    public static readonly BindableProperty ValueColorProperty = BindableProperty.Create(nameof(ValueColor), typeof(Color), typeof(LabeledValue), propertyChanged: OnValueColorPropertyChanged);

    private static void OnValueColorPropertyChanged(BindableObject bindable, object oldValue, object newValue)
    {
        ((LabeledValue)bindable).MyLabel.TextColor = (Color)newValue;
    }
}

When you use the LabeledValue control, you can either use the default behavior, which will respond to the App Theme or override it:

<!-- use default AppThemeBinding values -->
<components:LabeledValue/> 

<!-- permanently set the ValueColor to Orange-->
<components:LabeledValue ValueColor="Orange"/> 

You can find a working example for this in my MAUI Samples repository.