How to style the NestedMenuItem with css class?

[JensWinkler91]

I tried to style the NestedMenuItem using a css class (className) but it didn't work.

<NestedMenuItem
                    className={"ContextMenu"}
                    label="Add"
                    parentMenuOpen={this.props.open}
                >...

.ContextMenu .MuiPaper-root{
    background: linear-gradient(135deg, #333, #444);
    border-radius: 8px;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

It works for <Menu> (react material) but not for the nested NestedMenuItem. Can you tell me how I can achieve that?