<Styled @bind-Classname="@bigFont">
    label: big-font;
    font-size: 32px;
</Styled>

<Styled @bind-Classname="@div" ComposeBigFont="@bigFont" ComposeBigFontIf="@_showBig">
    label: white;
    background-color: white;
</Styled>

<div class="@div">
    Some text
</div>

<button @onclick="@onclick">Toggle Font Size</button>

@code {
    private string div, grey, bigFont;
    private bool _showBig = false;

    private void onclick()
    {
        _showBig = !_showBig;
    }
}