Basic example of programmatic ResourceDictionary configuration

App.cs

public class App : Application
{
    public App()
    {
        StyleUtils.ConfigureResources(Resources, new ThemeLight());
        
        ...
    }
}

Styles.cs

public static class StyleUtils
{
    public static readonly Thickness PagePadding = 16;

    public class ThemeLight : ITheme
    {
        // Material theming

        public Color ColorPrimary { get; } = Color.FromArgb("#FF0D4EB6");
        public Color ColorBackground { get; } = Color.FromArgb("#FFFFFFFF");
        public Color ColorSurface { get; } = Color.FromArgb("#FFFFFFFF");

        public Color ColorOnPrimary { get; } = Color.FromArgb("#FFFFFFFF");
        public Color ColorOnBackground { get; } = Color.FromArgb("#FF222426");
        public Color ColorOnSurface { get; } = Color.FromArgb("#FF222426");

        // Typography

        public Color ColorPlaceholder { get; }

        public ThemeLight()
        {
            ColorPlaceholder = ColorOnBackground.WithAlpha(0.5f);
        }
    }

    public static void ConfigureResources(ResourceDictionary r, ITheme theme)
    {
        SetTheme(r, theme);

        r.Add(new Style(typeof(NavigationPage))
        {
            Setters =
            {
                ThemeUtils.GetSetterDynamic(NavigationPage.BarBackgroundColorProperty, nameof(ITheme.ColorPrimary)),
                ThemeUtils.GetSetterDynamic(NavigationPage.BarTextColorProperty, nameof(ITheme.ColorOnPrimary)),
            }
        });

        r.Add(new Style(typeof(Page))
        {
            Setters =
            {
                ThemeUtils.GetSetterDynamic(Page.BackgroundColorProperty, nameof(ITheme.ColorBackground)),
            }
        });
    }

    public static void SetTheme(ResourceDictionary r, ITheme theme)
    {
        // TODO(jpr): Page.BackgroundColor is not respecting dynmamic resource changes...
        // see: https://github.com/dotnet/maui/issues/4516
        Theme.Current = theme;

        r[nameof(ITheme.ColorPrimary)] = theme.ColorPrimary;
        r[nameof(ITheme.ColorBackground)] = theme.ColorBackground;
        r[nameof(ITheme.ColorSurface)] = theme.ColorSurface;
        r[nameof(ITheme.ColorOnPrimary)] = theme.ColorOnPrimary;
        r[nameof(ITheme.ColorOnBackground)] = theme.ColorOnBackground;
        r[nameof(ITheme.ColorOnSurface)] = theme.ColorOnSurface;
        r[nameof(ITheme.ColorPlaceholder)] = theme.ColorPlaceholder;
    }

Theming.cs

public static class ThemeUtils
{
    public static Setter GetSetterDirect(BindableProperty property, object value) => new() { Property = property, Value = value };
    public static Setter GetSetterDynamic(BindableProperty property, string key) => new() { Property = property, Value = new Microsoft.Maui.Controls.Internals.DynamicResource(key) };
}