How can I dynamically apply Lightweight styling to NavigationView?

Changing the NavigationViewContentMargin dynamically, which determines the top margin of the NavigationView, does not change the appearance of the NavigationView dynamically.

Is there a way to dynamically change the top margin of NavigationView from the code?

top margin of NavigationView

You need to get the target Grid "NavigationViewContentMargin" is applied to. This Grid is named "ContentGrid".

The tricky part is that NavigationViews have 2 Grids with the name "ContentGrid". The "ContentGrid" we need is the one that is inside a Grid with the name "ContentRoot". BTW, the other one is inside "Settings".

First, we need to get the "ContentRoot" Grid, then get the "ContentGrid" we need.

Check this example:

MainWindow.xaml

<Window
    x:Class="NavigationViewTests.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="using:Microsoft.UI.Xaml.Controls"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="using:NavigationViewTests"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid ColumnDefinitions="*,Auto">
        <Grid.Resources>
            <Thickness x:Key="NavigationViewContentMargin">0,48,0,0</Thickness>
        </Grid.Resources>

        <NavigationView
            x:Name="NavigationViewControl"
            Grid.Column="0"
            IsBackButtonVisible="Visible">
            <TextBlock Text="Content" />
        </NavigationView>

        <StackPanel Grid.Column="1">
            <NumberBox
                x:Name="NavigationViewContentGridTopMarginNumber"
                Header="NavigationView Top Margin"
                ValueChanged="NumberBox_ValueChanged" />
        </StackPanel>
    </Grid>

</Window>

MainWindow.xaml.cs

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using System.Linq;

namespace NavigationViewTests;

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();
        this.NavigationViewControl.Loaded += (s, e) =>
        {
            this.NavigationViewContentGrid = GetNavigationViewContentGridInsideContentGrid();

            if (this.NavigationViewContentGrid is not null)
            {
                this.NavigationViewContentGridTopMarginNumber.Value = this.NavigationViewContentGrid.Margin.Top;
            }
        };
    }

    private Grid? NavigationViewContentGrid { get; set; }

    private Grid? GetNavigationViewContentGridInsideContentGrid()
    {
        Grid? contentRoot = this.NavigationViewControl
            .FindChildrenOfType<Grid>()
            .Where(x => x.Name == "ContentRoot")
            .FirstOrDefault();

        return contentRoot?
           .FindChildrenOfType<Grid>()
           .Where(x => x.Name == "ContentGrid")
           .FirstOrDefault();
    }

    private void NumberBox_ValueChanged(NumberBox _, NumberBoxValueChangedEventArgs args)
    {
        if (NavigationViewContentGrid is not null)
        {
            Thickness currentMargin = this.NavigationViewContentGrid.Margin;
            this.NavigationViewContentGrid.Margin = new Thickness(
                currentMargin.Left,
                args.NewValue,
                currentMargin.Right,
                currentMargin.Bottom);
        }
    }
}