private readonly ThemeState _state;

public MyClass(ThemeState state = null)
{
    _state = state;
}

private readonly ThemeState _state;

public MyClass(ThemeState state = null)
{
    _state = state;
}

services.AddSingleton<ThemeState>();

        Resources.Add("services", services.BuildServiceProvider());

services.AddSingleton<ThemeState>();

        Resources.Add("services", services.BuildServiceProvider());

public class ThemeState
{
    public CssBuilder ThemeCSS { get; set; }
    public event Action OnChange;

    public ThemeState() { ThemeCSS = CssBuilder.FromFile(ThemePath, ":root"); }

    public static readonly string WWWRootPath = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)!, "wwwroot");

    public static readonly string ThemePath = Path.Combine(WWWRootPath, "css", "theme.css");

    public void ApplyTheme(CssBuilder css)
    {
        ThemeCSS = css;
        OnChange?.Invoke();
    }

    public void UpdateTheme(string property, string value)
    {
        ThemeCSS.SetProperty(property, value);
        OnChange?.Invoke();
    }
}

public class ThemeState
{
    public CssBuilder ThemeCSS { get; set; }
    public event Action OnChange;

    public ThemeState() { ThemeCSS = CssBuilder.FromFile(ThemePath, ":root"); }

    public static readonly string WWWRootPath = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)!, "wwwroot");

    public static readonly string ThemePath = Path.Combine(WWWRootPath, "css", "theme.css");

    public void ApplyTheme(CssBuilder css)
    {
        ThemeCSS = css;
        OnChange?.Invoke();
    }

    public void UpdateTheme(string property, string value)
    {
        ThemeCSS.SetProperty(property, value);
        OnChange?.Invoke();
    }
}