How to use CSS isolation in Blazor via rendertreebuilder?

My component file MyComponent.cs

public class MyComponent : ComponentBase
{
    protected override void BuildRenderTree(RenderTreeBuilder builder)
    {
        base.BuildRenderTree(builder);

        builder.OpenElement(1, "div");
        builder.AddAttribute(2, "class", "my-class");
        builder.AddContent(3, "My component 1");

        builder.CloseElement();
    }
}

My css-file MyComponent.cs.css

.my-class {
background-color: red;}

css is not added to assembly