BlogPost5_Blazor_App.razor

<CascadingValue Value="_theme">
    <CascadingValue Value="@((Action)(() => _theme = (_theme == "light" ? "dark" : "light")))">
        <div>
            <MyButton Text="Click me!"></MyButton>
            <br />
            <ThemeToggler></ThemeToggler>
        </div>
    </CascadingValue>
</CascadingValue>

@code {
    private string _theme = "light";
}